From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Guide on photometer usage, brightness units, and exposure estimation.
| Identifier | ExFiles\Box 168a\3\ img525 | |
| Date | 1st May 1950 guessed | |
| The following table gives the relationship between some brightness units in common use and shows where they come with respect to the three ranges on the photometer. Approximate Relationships of Different Brightness Units (1 foot-lambert = 0.318 candles/sq. foot = 0.00221 candles/sq. inch = 3.43 candles/sq. metre). Photometer range | Log foot-lamberts | Foot-lamberts | Candles/sq. foot Low (red index) .. | 2.0, 1.0, 0 | 0.01 (1/100), 0.1 (1/10), 1 | 0.0032 (1/314), 0.032 (1/31), 0.32 (1/3) Normal (white index) | 0, 1.0, 2.0 | 1, 10, 100 | 0.32 (1/3), 3.2, 32 High (blue index) | 3.0, 4.0 | 1000, 10000 | 320, 3200 Values for interpolation between any of the above, the decimal place to be adjusted as required .. 0 | 1 | 0.32 .1 | 1.3 | 0.4 .2 | 1.6 | 0.5 .3 | 2 | 0.63 .4 | 2.5 | 0.8 .5 | 3.2 | 1.0 .6 | 4 | 1.3 .7 | 5 | 1.6 .8 | 6.3 | 2.0 .9 | 8 | 2.5 1.0 | 10 | 3.2 Other definitions used in Illuminating Engineering will be found in B.S. 230. Projection screen brightness. A valuable use of the photometer to the still or motion picture projectionist is in the checking from the projector of the screen brightness. B.S. 1404-1947 lays down the limits for 35 mm. film projection as 8 to 16 foot-lamberts with the projector running but with no film in the gate. For sub-standard film or lantern slide projection, where the room is often darker than the commercial cinema, experience suggests that a slightly lower figure may be permissible, but for really satisfactory projection quality, particularly for colour, the level should not be allowed to fall below 3 foot-lamberts. This applies to average films or slides. 22 Examples of exposure estimation The following examples illustrate subjects in which accurate exposure estimation with the S.E.I. Exposure Photometer is straightforward, but without which varying degrees of difficulty would arise. The examples include subjects having :— (a) long range, requiring accurate exposure. (b) extreme brightness distribution, requiring selective exposure. (c) controlled illumination where control of brightness range is necessary for satisfactory results. (d) low levels of illumination. It will be seen that there is nothing photographically abnormal in any of the subjects illustrated, yet they defeat existing types of exposure meter. A little experience will confirm that the S.E.I. Photometer surmounts all such problems with ease and indicates exposures which are uniformly reliable and accurate. 23 | ||