- 3 ~ 
has a spacing t equal t-o 20 urn and two symmetrical working 
orders q^- 2 and Op= -2. Transparent index grating 2 has a 
apacin 0 of 3 uu and three working spectral orders = 1 , 
P^- 0 and = -1. This combination of spectral oreders makes 
it possible to obtain light flux modulation period equal to 
4 um i not ho resultant zex'O spectral order* To provide for the 
use of the diffraction encoder in the measuring channel, ampli 
tude modulation of the light flux should be accompanied with a 
system of four phase-shifted signals for reversing the counter* 
This is attained by using the transparent diffraction grating 
with the phase-shifted fields and a unit of light—splitting 
wedges 3 * hue to such a design, four autocollimation images of 
the luminous element of illuminator 5 are produced on a light- 
sensitive surface of photodiode 6 in the focal plane of objec 
tive 4. Each of these images is formed as a result of diffrac 
tion of light beams in a certain zone of the grating and is gi 
ven a serial phase shift 90 degrees. The quantization of the 
photoelectric signal in steps of 1 um is accomplished by the 
electronic circuit. 
Investigations performed on stereocompara 
tor CKB-1 show that the maximum error of the encoder does not 
exceed 1 um. Highly accurate and stable results are fully ensur 
ed by very accurate diffraction gratings which possess a large 
resolution. A considerable modulation depth at a large gap 
between the gratings is due to compensation of phase errors 
.which sare usually encountered In the Eerranti system. 
Diffraction type encoders represent a kind of accumulat 
ing converters where coordinate information is obtained as an 
algebraic sum of elementary increments of discrete values. In 
such converters, coordinate information cannot be easily re 
stored if a failure occurs in the operation of a reversable 
counter. Some photogramnetric instruments developed in the 
Soviet Union use a linear photoelectric encoder which possesses 
both higher accuracy of accumulating.converters and stability 
of positional converters. The linear encoders are widely used 
in metal-wcrking, machines with digital control and on various