May, 1960 Commission I 175 
  
Fıc. 3. Checking that emulsion surface is perpendicular to centre collimator. 
errors and gives an accuracy better than one second. An autocollimating device has 
been added to the calibrator to ensure that the photographic emulsion is perpendicular 
to the central collimator for the exposures. This prevents tilts due, for example, to 
dust from appearing incorrectly as asymmetries. The calibrator with its accessories is 
shown in Figures 1, 2, and 3. 
A new comparator improves plate reading accuracy. The measurement of tan- 
gential distortion has become one of the routine tests. Calibration accuracy is now 
stated to be within 3 microns, based on a 99% confidence level. This applies to 
distortion measurements on the modern cameras with good image quality. 
For testing the deviation caused by filters and port glasses a new instrument with 
high magnification provides immediate direct readings to one second (Figure 4). With 
it changes in deviation over the area of filters and port glasses have been studied to an 
extent previously impractical. 
For routine measurement of film plane illumination and for determination of 
T-stops the camera is placed over a uniform diffuse light source and illumination is 
measured with a photoelectric photometer (Figure 5). 
A. change seems to be beginning from the traditional use of Kodak Aero Super XX 
film. Its use was necessary with the older cameras since there was a serious loss of 
resolution if the lens apertures were opened up to permit use of lower speed material. 
However the newer cameras show much less loss of resolution at large aperatures and 
also have better optical performance. Consequently films that give better resolution 
at somewhat lower speeds seem likely to be useful. Some practical use has been made 
of the finer grain materials and several experimental studies involving them are under 
Way.