plane, and in which the image forming rays are incident on the plate at an 
angle. Even if the Porro-Koppe method 
of measurement is employed, where the 
measuring system matches the image 
forming system as closely as is 
possible, errors can still occur if the 
plate changes shape for any reason 
between exposure and measurement. 
Perspective 
centre 
   
   
In general, the photographic plate 
will not-be flat. In addition, it may 
change shape between exposure and 
measurement for two main reasons: 
deformation may occur due to the forces 
holding the plate in the camera, and a 
change in relative humidity will cause re 
the emulsion to expand or contract point 
causing the plate to bow. 
Transverse 
= a error 
| 
| 
| 
  
   
  
Image plane 
   
Bowed plate 
The effects of this lack of flatness 
can be very great ( at 30 off axis the 
transverse error will be equal to half Fig. 3 
the size of the deviation from the 
image plane), and so steps must be taken to prevent, or correct for, these 
effects. 
Plate distortion due to humidity changes 
Stability of the plate with time, and with varying environmental 
conditions, must be checked. In particular, photographic emulsions coated on 
plates can absorb water from the atmosphere, so that if the humidity changes 
the emulsion either absorbs or loses water, and either expands or contracts. 
If the emulsion contracts then that face of the glass will be in compression, 
and the plate will distort. Also, any expansion of the emulsion will release 
the compression, and may even produce a tension. The form and extent of this 
distortion were determined by experimentation. 
‘Tesa’ 
displacement 
je A gauges 
  
  
  
Photographic 
plate 
Ball 
support 
  
  
  
  
  
  
  
A tB 
  
  
Fig. 4 
191