I welcome information from anyone who can add to what is presented here. Essentially, Ilford marketed a very early multiple rotary disc exposure calculator manufactured in three main designs (see below), various forms of Actinometer, exposure tables, three selenium cell photo-electric exposure meters. They also acted as distributors for the S.E.I. Photometer and Weston Master selenium cell exposure meters.

To view many early exposure meters, some similar to the ones illustrated and described here, take a look at Barry Levinson's impressively illustrated website. Barry lives and works in Israel.

For information relating to the ILFORD rotary disc exposure calculator I am indebted to Roy Phillips who once owned the aluminium version of the calculator (see below) and who provided the scan of the (maybe 1901) instruction booklet. I must also thank Richard W Holzman (who has previously assisted me with my Johnsons of Hendon and Wellcome Exposure Calculators) for his great expertise on all matters relating to exposure measurement. More recently Doug McKee has sent me information relating to announcements by the BRITANNIA WORKS Co.Ltd; Ilford, London (as Ilford Ltd was known, prior to 1901 - see Chronology) concerning the meter's availability and early manufacturing difficulties.

The following chronological information relating to the Ilford Exposure Meter have been extracted by Doug McKee from issues of 'Photographic Scraps' a free monthly news sheet produced by the Britannia Works Co. Ltd (believed to have been printed from around August 1889).

August 1st 1892. ..... The "llford" Exposure Slide Rule or Exposure Meter, specially designed for use with the various rapidities of "Ilford" plates, by Professor A. J. Scott, of Dublin. Professor Scott is known as one of the most careful investigators we have, and was the first to give to the world the table of the varying actinic values of light at different seasons of the year and hours of the day. This table has been used by all writers, &c (sic; perhaps this should read "etc") as the standard, and Professor Scott has given the most careful study to the working out of this exposure slide rule for us. It is based on the experience of many years, and its efficiency proved by a long series of actual tests. The instrument will be made in the most careful and substantial form, so that it will bear handling, and will be convenient for the pocket and, lastly, it will be more economical in price than any exposure meter yet introduced.

September 1892. This is in the manufacturer's hand, and will be ready in a few weeks. A scientific instrument for the calculation of exposures of Ilford Plates.

October 1892. It is approximately the same weight and thickness, and can be as easily carried in the waistcoat pocket, as a crown piece. Its value is intrinsically far greater than that coin, and yet its price is only 5/-, post free 5/2. Will be ready shortly.
(the crown was a Bitish coin valued at 5/- = 25p, first minted in the reign of Henry VIII, it was last minted in 1965).

November 1892. We have placed the manufacture of these in the hands of a well-known Birmingham metal worker, and have had the first trial stampings from the dies. These please us very much, and the instrument promises to be even neater and better made than we expected. Six separate dies and stampings are used, and this, with the requisite fitting and finishing, necessarily takes time, so that we are not yet able to announce that the meter is ready. Before we do so, we shall get a considerable stock in hand, for the orders already received are numerous, and we do not want to put them on the market until we have sufficient to supply all demands, and thus prevent disappointment.

December 1892. The manufacture of these is being rapidly pressed forward, and we are getting a stock in hand, as already advised. We hope another ten days or so will see them ready to be issued. The demand is much larger even than we anticipated, and the care bestowed on their production renders the output somewhat slow.

February 1893. After spending three months in making these, the metal stampers have found aluminium too stubborn and variable to allow of a really uniform and high-class article being made from it. Brass, or any other metal, is too heavy, so that we have decided to make the meters in ebonite. This will necessitate new dies, new presses, etc., and we must therefore ask the indulgence of our friends for a little further delay. It will not be long.

July 1st 1893. We have not said anything about this instrument in our recent issues because we wished to have it actually finished and ready for sale before we spoke of it again, in order to prevent further disappointments from possible delays.
In working out the details of any good instrument of precision there are always so many unforeseen difficulties to contend with, that our first announcement on the subject was, we admit with regret, very premature. The time spent in waiting has, however, given us ample opportunity of perfecting the instrument, and we say with considerable pride that it is a much superior article in every respect to the one we first planned. The ILFORD EXPOSURE METER IS NOW READY, PRICE 5s.; POST FREE, 5s. 2d (26p). It can be had from all photographio dealers throughout the world, or direct from us. It is made in the best ebonite, of two colours, for convenience of distinguishing the several factor circles.

As can be read above, although first announced in August 1892, due to manufacturing difficulties it wasn't available until July 1893, by which time its material of construction had changed from the originally intended aluminium to ebonite (one of the earliest forms of plastic). It was 3 inches (75mm) diameter, weighed 1 ounce (28.3grammes) and cost 5s (25p). Alongside is the meter in its ebonite form, made of black and red ebonite discs. By October 1893, the Brittania Works announced that the third 1000 had been manufactured ready for delivery.

The instrument consists of three movable circles or discs, which revolve freely on a central pivot, and two fixed discs at the extreme rear and at the front. The front (fixed) disc shows information relating to the sensitivity to light of the plate or film in use. The next down shows information relating to the intensity of sunlight dependent upon the date (called the Date Circle - used in conjunction with the Date Table on the back of the meter - scroll down, see RHS). The third is the Diaphragm Circle relating to the lens aperture (i.e. f stop) in use. The fourth disc is the Subject Circle relating to the type of subject being photographed. The rear (fixed) disc is the Exposure Circle which shows the required exposure in fractions or whole seconds, minutes or (even) hours (!).

This ebonite version proved difficult to use because the markings were not distinct. So the design eventually (1899) reverted to aluminium.

The instruction booklet alongside gives a full description and method of use. Click here or on the icon to download it as a pdf. The footnote F1 appears in the lower left of the front page, suggesting it was printed in June 1901; scans courtesy of Roy Phillips, see below. Although this booklet was provided with a later (post-ebonite) aluminium version (see below), the size (3" dia) and weight (1oz), shown on the inside of the back page, still refer to the previous ebonite version.

The Calculator was patented by Professor John Alfred Scott and John Howson of the Britannia Works (subsequently Ilford, Limited). The diagram alongside is taken from the original 1892 patent, which can be viewed as a pdf here.

Richard W Holzman was surprised to see that the instruction booklet (see above) contains the name Professor A.Alfred Scott instead of Prof.John Alfred Scott. Richard believes this is the same man named as Dr.J.A. and John A. in various articles dating to the 1880 and 1890s, where he is associated with a table of comparative exposures. He had articles in the British Journal of Photography Almanac from 1886 for many years.

Doug McKee reports that Richard is exactly right in his surmise that the inventor's name is wrongly given in the 1901 instruction booklet.

Doug has a copy of the same booklet but pre-dating the 1901 issue (see the two images, below). This correctly names Professor John Alfred Scott as the inventor. The error pointed out by Richard W. Holzman must have been a typing error in the newer reprint.

As confirmation of the earlier date of the copy owned by Doug McKee (possibly 1893) notice that in the earlier copy the company name is the original 'The Britannia Works Co., Ltd', rather than (from 1901) 'Ilford, Limited'. The earlier version of the instructions also includes the BJPA review of the calculator, as can be read (in full) by clicking the hyperlink to the July 1893 'Photographica Scraps' magazine. The July 1893 announcement in 'Photographic Scraps' of the Rotary Disc meter and the full British Journal Almanac review, can be read here as a pdf (courtesy of Doug McKee).

Roy Phillips owned the aluminium version which can be seen here, first introduced in March 1899.

This version has markings which are far more legible. The scans here and below are from Roy.

Richard W Holzman reports that in 1906 a four disc version was on the market covered by a different patent (dated 1905) by Roland S. Potter, a research chemist at Ilford. This four disc version was available in aluminium (original price 5s = 25p) or heavy paper (price 1s = 5p; for picture, scroll further down) to at least 1929 and perhaps later. The original patent can be viewed as a pdf here.

Doug McKee sent me a pdf of the 1907 BJPA review, but now (March 2020) that BJPA entry has been updated by scans from Richard W Holzman. To view as a pdf click here.
Richard has also sent a scan from the 1906 BJPA. He comments "interesting how the text is the same in both". To view it as a pdf, click here.

Simon Spaans has kindly sent me the scans below of his 4-disc aluminium calculator.

A scan by Simon Spaans of the inside rear cover of the 4-disc version instruction booklet. Notice that this version is 2½" diameter (63.5mm) rather than the 3" (76mm) diameter of the ebonite 5-disc version (see above).

Simon says that "In the leather case for the disc, the (Dutch) owner had written a date: 5-2-1910, almost a hundred years ago!"

David Bayliss has been in touch (June 2017) to say that he also owns a card version of the 4-disc calculator, the same as the upper (earlier) of the two owned by Simon Spaans (above). He also has the instruction booklet and says:

"Apart from ‘1907’ pencilled on the cover, presumably by a previous owner, the booklet is not dated. However it does advertise the Ilford Manual of Photography at 1s 0d, which might give a ‘before’ date. The booklet is applicable to the aluminium version AND the card version indicating that they were, at least for a time, being produced simultaneously".

A scan of David's meter (kindly provided by David) is shown to the right.
The central letters, denoting the type of Ilford sensitive plate, are defined above.
The weather abbreviations on the 2nd scale from the top refer to:
S, WC = Sunny, White Clouds
S, BS = Sunshine, Blue Sky
C S = Cloudy Sky
D = Dull
V D = Very Dull

Some of the pages of the instruction booklet can be downloaded as a pdf here.

The Royal Photographic Society's Diary for 1925
In 1925, the Royal Photographic Society (RPS) produced their own version of a photographer's diary entitled "The Royal Photographic Society's Diary for 1925". It was priced at 2 shillings (2/- = 10p).

The Diary contained an exposure meter based upon the Ilford rotary disc design. The Preface ends "The Council desire to express their indebtedness to Mr. A. C. Banfield, F.R.P.S., and to Mr. Olaf Bloch, F.I.C, F.R.P.S.. for their invaluable assistance in contributing articles and to Messrs. Ilford, Limited, for the permission to use the Ilford Exposure Meter (from which it has been adapted) as the basis of the Diary Meter.

The RPS diary can be viewed by downloading the pdf here, courtesy of Richard W Holzman. As far as is known, no other RPS diaries were published.

The following is taken from:
http://www.answers.com/topic/exposure-meter?cat=technology.
"Actinometers were an early form of incident light meter that relied upon the darkening of a piece of photosensitive material. The time taken to reach a specified tint, marked on the body of the meter, was the basis for exposure calculation: some meters were fitted with a chain, allowing their use as a pendulum to also time a lengthy exposure. Their main disadvantages are sluggishness in use and a reliance on a consumable metering medium."

For much of the story and the pictures (alongside and below) of the Imperial actinometers and exposure calculators, I am indebted to Richard W Holzman. The pictures alongside are the covers of two Imperial Dry Plate Co.Ltd handbooks for the years 1907 and 1910, both within Richard's collection.

The Imperial Dry Plate Co. Ltd of Cricklewood, London, NW2, introduced an actinometer in 1901 as a card with an actinometer and calculator on one side and the instructions on the reverse. The original British Patent 3873 of 1901 can be viewed by clicking here.

Left and below is a version produced somewhat later, the H&D scale runs to 600 and the two sides of the original card are mounted within a half-fold green wallet.

Note the cover price of 1/- (1 shilling = 5p)

Imperial also introduced another device for exposure determination in 1901, called the 'Reckoner'. This was based upon a similar sort of sliding scale calculator as the actinometer (above) but exposure determination did not rely upon the actinometer method. It relied, instead, that most photographs were (still are) taken of 'usual' subjects under 'average' weather conditions, whereby a sliding scale calculator is adequate to arriving at an appropriate exposure. Hence, the need for exposing actinometer paper was avoided under these 'every day' photographic conditions.

As with the original 1901 Actinometer, the first form of the 'Imperial Exposure Reckoner' was a double sided card, as shown below. These scans are of an 'Imperial Exposure Reckoner' sold for use in Australia, so the Table of Light Values is intended for latitude 30 degrees south! My thanks to the current owner, Andrzej Jablonski, for these two scans, front & back.

By 1907, the 'Reckoner' had become a half-fold wallet (see below). This one has a red cover. It is likely that the actinometer version of the 'Imperial Exposure Meter' had also become a half-fold design by this time.

For larger views of an early, and also a subsequent version, of the 1907 Reckoner, click here or the image below

The picture below, of a double sided card version of the Imperial Exposure Reckoner (as the one shown above) has been taken from the ebay auction site. Hence its 'true' colour is unknown.
Notice that there are two independent sliding strips enabling the Reckoner to perform its exposure calculation. They are visually 'linked' by the centre strip and its crossing lines.

The British Journal of Photography for April 7th, 1911 (p270) announced the arrival of a second actinometer to supplement the original, with the new arrival being for use in Dull Light. The BJP announcement can be read here. The original was now renamed the 'No.1 for Bright Light' and the new was called the 'No.2 for Dull Light'. Individually, each was still priced at 1/- (5p).

In the Shire book entitled "Photographic Accessories 1890-1970" by Robert White, the No.1 for Bright Light is shown in a red cover and the No.2 for Dull Light is shown in a green cover (p28). However, a No.1 owned by Richard W Holzman has a green cover (extreme right). The cover colours of the other illustrated versions has been stated, see their adjacent text, but there seems no consistency.

The light sensitive paper used by the actinometers was the same for both the No.1 and the No.2 meters, the difference was only to do with how dark the paper needed to become in order to indicate the correct exposure. The colour comparison disk, surrounding the hole through which the light sensitive paper was exposed to light, was lighter coloured in the No.2, such that the paper did not need exposing to dull light for an excessive time in order to achieve a colour match.

A pdf of the Instruction Booklet for the No.1 can be downloaded here or click on the icon.

A pdf of the Instruction Booklet for the No.2 can be downloaded here or click on the icon.
Another offering in 1911 was the 'Duplex' which had both the No.1 and No.2 actinometers in a single three-fold case (see right and below). The one illustrated has a black cover.

The British Journal of Photography for March 14th, 1913 (p213) announced an Imperial 'Combined Meter' which consisted of the No.1 'For Bright Light' actinometer and the Reckoner in a three-fold case. The announcement can be read here. The price, including instruction booklet and light sensitive actinometer paper, was 2s/6d (12.5p). Richard has seen a red covered 'Combined' (perhaps like the one shown below, owned by David Bayliss.

The idea being that the exposure for most average subjects could be assessed using the Reckoner without need for exposing light sensitive paper in the No.1 actinometer. But the actinometer was there in case the photographer met unusual conditions which he wasn't confident he could assess using the Reckoner. "The new imperial meter fulfils the conditions in both cases".

David Bayliss (see his very early Imperial Exposure Meter above, near the top of this section) has now (June 2024) sent the pictures to the right and below, showing a nice maroon bound example of the Imperial Combined Exposure Meter and Reckoner, which consists of the 'Imperial Exposure Reckoner' and the 'Imperial Exposure Meter No.1 For Bright Light'.

The Reckoner and Exposure Meter are all bound together in a similar fashion to Imperial's Duplex Meter.

The right hand picture shows the Meter, the original box, the instruction booklets and the light sensitive paper, which is used to assess the lighting level when the Table of Light Values is insufficient guidance. Usually, in bright light, the use of the light sensitive paper is unnecessary.

The two pictures below show how the single triple fold 'book' could be opened to reveal either the No.1 Imperial Exposure Meter for Bright Light or the Imperial Exposure Reckoner. The red front covers on the pictures below seem to contradict the two white pages that are alongside. However, that is to do with how the triple foid is arranged. In use, when either red cover is opened, the two white pages that are revealed are the ones on the 'other' side of the ones in the pictures below - hence correctly corresponding with the red cover.

In the bottom left hand corner of the two instruction booklets shown to the right, are the numerals '619' (Exposure Reckoner) and '1020' (Exposure Meter). These are believed to stand for June 1919 and October 1920, respectively. David comments how the booklets are over-stamped 'Prices Advanced', possibly a subtle way of saying 'Prices Increased'.

The Imperial Dry Plate Co.Ltd of Cricklewood, London, NW2, came under Ilford Ltd control during the period 1918 to early 1930s when many of the smaller UK photographic firms amalgamated with the larger organisation. See the Ilford Chronology, 1930, for more information, but it is imprecise.

The British Journal of Photography (BJPA) for September 1936, page 18, says "Users of the well-known Imperial exposure meters and reckoner, will now find them under the name of Ilford, unchanged in principle or appearance, but carefully brought up to date and adjusted to give absolutely uniform and reliable results under all conditions. Those who use 'sensitive tint actinometers' will find these meters - one for bright light, and the other for dull light and interiors - most convenient and reliable. The price is 1s/6d (7.5p) for either meter or reckoner, or 3s/6d (17.5p) for the Duplex meter combining the two single meters."

Left (below) is one of the original Imperial Exposure Meters and to the right is the Ilford equivalent, post-1936. This scan has been sent to me by Jeff Gould. The largest f-stop has been extended from f4.5 to f2.5, and the film speed range is increased form H&D 600 to H&D 6,000.

Richard W Holzman comments that the 1936 Ilford versions of the Imperial actinometers may not have been sold for long as he has never seen any apart from the photograph shown alongside, taken from Photographic Science & Engineering (PS & E) magazine for March / April 1959, page 79.

Interestingly, although this one should be the same as the No.1 Bright Light version sent to me as a scan by Jeff Gould, see above, the one that was pictured in the 1959 PS & E magazine has a film speed range that only runs to H&D 1200, rather than H&D 6,000. The f-stop range is the same, running from f2.5 to f64. So the PS & E version seems to 'sit' between the earlier Imperial No.1 Actinometer and the Ilford No.1 owned by Jeff Gould. Perhaps the one owned by Jeff is a later version, when film speeds had been extended, whereas the PS & E image is a true 1936 example ?

H&D 6,000 corresponds to the Ilford film speed Group G, which didn't appear on Ilford's selenium cell exposure meters until the Model C in 1949 (see below). If this is the case, then the Ilford actinometers were presumably sold for at least 13 years, from 1936 to 1949, and perhaps into the early 1950s.

In the same 1936 BJPA reference as above, the text continues:

"Messrs. Ilford have also issued a Selo speed card, which gives the daylight speed of all Selo films in DIN, Scheiner, H&D (English & Continental), Weston, Burroughs Wellcome, and Justaphot. All who use Selo films should secure a copy of this card."

Richard W Holzman has kindly supplied me with a pdf file of this card, the front cover of which can be seen left. To download the pdf, click here or on the cover image.

The footnote on the final page reads 'K 36', implying November 1936.

Although not strictly an exposure calculator, the 'Ilford Exposure Tables' booklet is included for completeness.

Unlike the Selo card, above, these Tables require no calculation in their use. The booklet is simply a collection of tables showing (directly) the recommended exposure under a wide range of conditions and times.

Richard W Holzman has supplied me with scans of all the pages which make up the Tables. The cover is shown (far left) and below is an example of the pages. A full version can be viewed as a pdf by clicking here or on the image left.

A footnote on page 1 (A 34) suggests these Tables date to January 1934.

There were three of these meters, all physically similar but with their chronology traceable by the Ilford Speed Group lettering system. The first meter had speed graduations A-E (4-64ASA), the second, Meter B, was also initially marketed with A-E but subsequently its speed range was extended to the range A-F (4-125ASA). The third, known as Meter C, had A-G (4-250ASA). The instruction booklet for the Meter B indicates that the film range A-E was later extended to include the speed rating 'F' due to the increasing speed of Ilford's Hypersensitive Panchromatic plates and films. The Ilford Speed Group system was eventually extended to cover A-H (4-500ASA) in order to include their fastest emulsion (one time fastest in the world) HPS, from 1952.

Richard W Holzman confirms that these meters were made by the UK company known as "AVO" (previously known as the Automatic Coil Winder & Electrical Equipment Co. Ltd; ACWEECO; started in 1923). In 1937 their address was Windsor House, Douglas Street, London, S.W.1.

Andy Gilham, one of the mechanical designers at Megger Instruments Limited, Archcliffe Road, Dover, Kent CT17 9EN, the company that was formerly AVO Ltd, has e-mailed (November 2016) to tell me that AVO moved to Winder House in 1930, having previously spent a short time at Rochester Row, London, and before that in Wellington House, Buckingham Gate, London (where the company began).
Andy says that Megger Instruments have been manufacturing in Dover since 1966 (when they moved from Winder House, London). During their recent 50th year celebration they held an Open Day and Andy collected a lot of historical documents for display. He was given an Operation Manual for the Universal AVO Exposure Meter and "I thought you might like the PDF of the guide". To download that guide, click here.
Also, download from here the pdf of another early AVO meter owned by Ian Grant.

Further from Andy Gilham (April 2020) is that the original Avo Exposure meter was launched in 1935, based on design patents by Hugh Sutherland Macadie. The design was cased in a Bakelite housing that very much resembles the Avometer at the time, with the ‘kidney dish’ shaped window. Avo dropped this housing style for the Exposure Meter Model 2 the following year and Ilford continued using it, but still manufactured by Avo/ACWEECO.

Andy is sure, from the dismantled view of the Ilford Model A (see below) that it is of Avo/ACWEECO style. He derives this from the design of the 'movement' and more importantly the serial number on the scale plate, which reads 1361-86. This instrument was manufactured in August 1936 and it was 1361 instrument to be sold (in total).

The date is in the last 2 digits (86). The first digit is the month (August) and the second is the year, where the decade is implied (1936).

Meter A (not its official name, but used here to distinguish it from the later photo-electric Meters B and C, below) was marketed from 1935.

The 1936 BJPA article, page 291, (click here for a pdf original) reads:
"THE ILFORD PHOTOELECTRIC EXPOSURE METER. (Sold by Ilford Limited, Ilford, London, E.)
This is a really British meter - made in England by a leading firm of electrical measuring instrument manufacturers in collaboration with Messrs Ilford, and scaled in terms of British materials: Ilford plates and films. It is a handy little pocket instrument, in a neat leather ever-ready case. Operation is simplicity itself. A disc is turned once and for all to the speed of the plate or film - these are divided into groups; no speed numbers are used - the meter is pointed at the subject and a second disc turned to the desired stop, when the pointer will indicate the required exposure direct.
In addition to its use with ordinary materials, the meter has been specially calibrated for use with Dufaycolor roll film, flat film and 16mm cine film, thus ensuring the maximum possible accuracy in exposure with this material. All Dufaycolor workers should possess one.
The double cell is deep set, and therefore includes only the actual camera angle. The price of this handy little meter which, it is safe to say, from the reputation of the makers, is robust and reliable as well as extremely sensitive, is £3 3s 0d (£3.15p)."

The meter had a leather case with wrap-around twin press-stud closers. The instruction book is available to view as a pdf, courtesy Richard W Holzman. This issue of the instruction book is code dated H 35, meaning August 1935.

Click here or on the icon to download the pdf.

The high quality scans of Meter A shown below are all courtesy of Richard.

Note the green, black & red arrows on the outer disc and the similarly coloured scales in the meter needle window. Their mode of use is explained in the Instruction book (see above) and also referenced in the brief instructions, below.

The Ilford Speed Group lettering scale from A to E is clearly visible (see left).

Richard has partly dismantled a Meter A to understand how its triple sensitivity range worked.

He comments "The three ranges on the meter are accomplished by means of a fine wire wound rheostat connected to the outer dial."

The rheostat slider can be seen at the bottom of the picture.

From the number on the exposure scale, Andy Gilham (of Megger Instruments - see his further information in the opening paragraphs above; ILFORD photo-electric exposure meters - made by AVO) can see that this instrument was manufactured in August 1936 and it was the 1,361 instrument to be sold (in total).

The date is in the last 2 digits (86). The first digit is the month (August) and the second is the year, where the decade is implied (1936).

Brief 'reminder' instructions for use are moulded into the back of the (presumably) Bakelite casing of the meter. They are transcribed here:

INSTRUCTIONS:
Depress and turn disc bearing speed group indication letters A, B, C, D, E until arrow on centre boss points to (speed group) of material used.
Direct window of meter towards subject, avoiding the inclusion of too much sky.
Still Cameras
Method 1. Rotate outer disc until needle points to exposure desired on one of the scales and read stop number against arrow of corresponding colour on outer disc.
Method 2. Rotate outer disc until red, black or green arrow points to stop number desired and read exposure against the needle on scale of corresponding colour.
Cine Cameras, for shutter speeds of 1/32 second, use Method 1, above, setting the needle on the special mark (1/32) provided, reading stop number against the black arrow. For other shutter speeds, use Method 1 above.
Made in England, Provisionally Patented.

"AVO", the makers of the Ilford exposure meters, advertised their own design around the same time that the first Ilford meter went onto the market. The one illustrated below is being sold at the same price as the Ilford version. For more information on the AVO company (now Megger Instruments), scroll up.

To view pictures of an early AVO meter owned by Ian Grant, and download its instruction manual, click here.

This advertisement is taken from the October 1937 issue of The Miniature Camera Magazine (inside front cover, pii). Although with some physical similarity, it is not the same.

Interestingly, the Miniature Camera Magazine for August 1937 may carry the first mention of an exposure meter working on the incident light basis of exposure measurement. The research for this technique is ascribed to Mr.P.C.Smethurst who called his invention the 'high-light' method. It was first described in the Amateur Cine World magazine.

Mr Smethurst and AVO got together on this and AVO produced a meter called the "Avo-Smethurst High-Light Meter" ~ see the insert in the right hand advert here. The Avo-Smethurst differed from the conventional AVO meter by having an opal glass plate immediately in front of the photo-cell. It appears this was a permanent feature, not detachable as became the norm with e.g the Weston Invercone, enabling the same meter to be used for both reflected and incident light readings.

Shown here are two versions, an early one (see left - courtesy of Richard W Holzman) which bears the Ilford Speed Group letters A-E (as the Model A), and a later version (see below, maybe post-WW2) which bears the Ilford Speed Group letters A-F. The later version has lettering which is far more visible than the earlier version.

The original instruction booklet (on its page 8) shows that, even at the time the booklet was produced (March 1938), the Ilford Speed Group range had already increased to include the letter 'F', necessitated by "the recent increase in speeds of Ilford Hypersensitive Panchromatic Plates and Films". It goes on to suggest that emulsions of speed rating 'F' should be treated as if of speed rating 'E' but given half the indicated exposure. The 1938 instruction booklet can be viewed as a pdf by clicking here or on the booklet icon.

The Model B was a dual range meter, having a red dot and a green dot. The method of use was to align the appropriate dot with the meter needle (not visible in the picture, below). To assist with this, there are raised lines in the casing between the central dials and meter window which align with the marks on the meter scale. An internal baffle plate moved in front of the cell (internal the meter's casing) when the red dot was used.

The meter was reviewed in the British Journal of Photography (BJP) for July 23rd 1937, p474 and the text is transcribed below the pictures - click here for a pdf of the original article. The original text of this article has some errors which confuse the method of use of the meter, so below I have added (in parenthesis) my understanding of what was actually meant.

This is the box in which the early version (as above) of the Ilford Photo-Electric Exposure Meter Model B was packaged for sale. Picture courtesy of Doug McKee.

The paddle steamer logo is the last version used, phased out during 1945, prior to the Model B meter update shown below.

The picture below is of a 1949 Meter Model C (picture provided by David Muggleton). The Ilford Speed Group lettering A to G is now accompanied by an Ilford Scheiner logarithmic speed rating. The instruction booklet says "the latter, which have frequently been referred to as European Scheiner degrees, are approximately one unit higher throughout than the corresponding B.S.I. logarithmic exposure indices".

This meter is single range, each Light Value on the scale under the meter window being twice the brightness of the previous numbered Value. The Light Value appropriate to the subject is read off from the meter needle indication and transferred to the central scales for calculating exposure. At this time, HP3 film was only a speed rating of 125ASA (Ilford Letter F, Scheiner 32) presumably to maintain sufficiently fine grain within small negatives. Plates, being larger and hence requiring less enlargement, were faster, being Ilford Letter G, Scheiner 35.

Scroll down to read the 1949 BJP and 1950 BJPA reviews

The original instruction booklet for the Meter Model C can be viewed & downloaded as a pdf by clicking the picture.

Review taken from pages 226-227 of The British Journal Photographic Almanac, 1950.

THE ILFORD PHOTO-ELECTRIC EXPOSURE METER (Model C) (Ilford Ltd., Ilford, London)

This new Ilford meter is contained in a black plastic case 2in.x2¾in. by rather under 1¼in. thick. The photo-cell is deep-set, giving an acceptance angle corresponding to that of a reasonably wide angle camera lens. Exposure calculation is effected by a single disc, which is very smooth running, with just enough milled edge projecting for "thumbing" round but not enough for accidental turning. This meter is of the type in which an arbitrary light value number is indicated by the pointer, and this number, as engraved on the case dial, is set against the speed of the material on the disc dial, the exposure then being read off direct, on the disc, against the scale of stops from f/1 to f/32 engraved on the case. The scales are in very clear white-filled engraving that shows up brilliantly in any light against the black of the case. This is not a double range instrument; its single scale, apart from the lowest readings, is almost uniform and covers a total range of light values of 6,000:1. The scale covers exposures from 1/2,000th sec to 1 minute; stops f/1 to f/32, and film speeds 17degrees to 35degrees. These are on the Ilford scale, and are approximately 1 degree higher than B.S. (logarithmic) numbers. Ilford groups (A-G) are also engraved on the disc.
On the back of the instrument, factors are given for the conversion of the "light values" read directly from the pointer into illumination foot candles, so that the instrument can be used as an illumination meter.
Excellent instructions are supplied with the meter, which will well repay careful study; an explanation is given of the blotting paper highlight method as applicable both for reversal materials and for dull lighting conditions; the same method is extended to artificial light, and factors are given for the three principal classes of material- hyper-pan, normal pan, and ortho; and for very dim artificial light, the light source method (pointing the meter direct at the principal light source) is given, again with appropriate factors.
The price of the meter, complete with neck cord, is £5 10s. 6d., plus £2 8s. 0d. purchase tax. A case for the instrument, of medium brown leather with a grained surface, is priced at 8s. 0d., plus 5s. 3d. purchase tax.

The S.E.I. (Salford Electrical Instruments) Photometer was distributed by Ilford Ltd. from 1948 and (in the U.S.A.) by The Zoomar Corporation. Clicking the following link takes you to a 'Shutterbug' web page written by Roger Hicks in October 2002. Roger fully describes the S.E.I and I will not repeat his work here. Essentially the Photometer is a comparative method of light intensity estimation, whereby a standardised light intensity, created by an internal lamp, is matched by eye to the light intensity of the subject. It works over a narrow angle of view so can take very detailed 'spot' readings. It can be a very accurate, but slow, exposure metering process.

My thanks to Huw for displaying the SEI instruction manual and repair manual on his website.

Newnes Photographer's Pocket Reference Book, published in 1955, contains (page 70) a description of its method of use.

British Sangamo was founded in 1921 as the United Kingdom subsidiary of the "Sangamo Electric Co." of Springfield, Illinois, USA. They initially manufactured integrating kilowatt hour meters and later expanded their activities to include industrial time switches.

In 1936, British Sangamo acquired the "Weston Electrical Instrument Co." of Surbiton, Surrey. This company was the UK subsidiary of the "Weston Electrical Instrument Corporation", of Newark, New Jersey, USA, a company founded by Dr. Weston (a pioneer in the field of electrical measurements), and recognized as one of the worlds leading manufacturers of electrical measuring and associated equipment. A detailed history of the Weston company is available here (click on 'About', top of left hand side list). The two companies were combined to form "Sangamo Weston", which designed and manufactured a wide range of electrical measuring and control equipment. Amongst these were the famous selenium cell Weston Master Exposure Meters.

Sangamo Weston Limited were based at Great Cambridge Road, Enfield, Middlesex, N.London, about 10miles North East of Ilford, so it perhaps makes geographical sense (though possibly just coincidence) that Ilford Ltd became UK distributors for Sangamao Weston Master exposure meters from the Weston Master 1 in 1947 (reviewed in the BJPA for 1948) through to the Weston Master 5, starting in 1963. IIlford's main involvement seems to have been in distribution and advertising. However they did provide additional accessories, like cases. It's possible Ilford were not involved with the earlier Westons because they had their own range from Avo (see Ilford meters 'A', B & C, above).

Doug McKee has kindly sent me the following images.
To the left (below) are show the reverse of a Weston Master 1 and 5 meters (Weston Masters 2, 3 and 4 all bear similar markings). Look closely and both can be seen to bear the information "Distributed by SANGAMO WESTON LTD & ILFORD LIMITED". The use of Limited, written in full when associated with the company name Ilford, is probably a 'throw back' to the 1901 requirement of the Ilford Urban District Council (see my Ilford chronology) at the time when the Britannia Works Co. Ltd first changed its name to Ilford Ltd.

Weston Master 1 (left) and Weston Master 5 to the same scale.

Two Ilford branded cases for the Weston Master II. The slimmer case (right) for just the meter (see below, right) and the larger to accommodate the meter and its incident light invercone and baffle (below, left).

Richard W Holzman has informed me of yet another Ilford exposure meter, though this one (best we know) never went into production. Our only knowledge of it is from its GB Patent 634,831 with an original application date of 17th December 1947, a 'date of filing complete specification' of 16th December 1948 and a 'complete specification published' date of 25th March 1950. The novelty claimed by the patent was for a photo-electric exposure meter constructed on a design similar to a wrist watch with the intention that it would be worn on the wrist for convenient use without needing to be hand-held. The joint proposers were Geoffrey Bond Harrison and Ilford Ltd, both of 23 Roden Street, Essex, UK.

The patent can be viewed here.

The idea of a wrist mounted exposure meter was not, in itself, novel. The novelty was in the use of a photo-electric cell within such a design. Previous wrist mounted meters were of the 'Visual' type; 'Visual' meters are pointed towards the subject and the user looks through the meter and views a series of letters or numbers behind an increasingly opaque (semi-transparent) screen. The letter or number that is just visible is used to indicate the amount of ambient light falling on the subject and, hence, the exposure required.

The Ilford patent referred to a true photo-electric exposure meter, with rotating scales forming the 'watch' face of the meter and providing means to calculate the exposure based upon the needle movement of a microammeter. It is unknown why this wrist-mounted meter never went into production. Perhaps 'AVO', who made Ilford's meters, did not consider it a viable concept.