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TABLE II LENS DIAMETERS FOR KINEMA STUDIO ILLUMINATORS
Lamp
Rating
Lens Diameter
Incandescent
500 watts
6 in.
Incandescent
2 KW
10 in.
*Incandescent
5 KW
14 in.
H.I. Arc
65 amps
Sin.
*H.I. Arc
120 amps
14 m.
H.I. Arc
150 amps
20 in.
* These lenses are of the same diameter but different prism formations.
1.3 Optical System for Illuminator Lens
It is not necessary in this paper to discuss detailed points in design of a prismatic plate lens. There are two main problems : (a) Design of prisms.
(b) Diffusion on back of lens.
/•'
A
/
I
LIGHT
1
SOURCE
1
\
\
\
V^
140° 150° 160° 170° IBO° 170° 160° 150° 140°
/
— -T~^L NA\ \/\\
130°
3^sJV)\^\\
120°
IIO°
100
90°
8d 70°
60°
50°
\S
^^rv^W/
130
120°
110° IOO° 90°
80 70°
60'
SO"
40
30 20 IO O IO 20 30 40
Fig. 4. Optically equivalent diffusing fittings
Fig. 5. Polar curves of diffusing fittings
Fig. 6 shows the main task which the lens must perform. If P is a section of correctly designed plate and A and B the extreme positions of the source then ray Aa must spread to the required amount whilst rays Bb must have only a smal divergence. Each of these rays is, of course, associated with a small cone of light depending on the source size and its distance from the lens as shown in the lower half of the diagram. If the lens is designed to have its best performance with the source at A — that is to have a perfectly uniform distribution — then it is found that with the source at B the light is not sufficiently concentrated. If the lens is designed to give maximum concentration with the source at B, the spread position will be found to show a dark centre. The art of the designer is to get a satisfactory compromise between the performances at these extreme positions.
Let us suppose that we have a lens designed correctly for source position A, that is to give a uniform wide-spread beam. This can be done by obeying a simple mathematical formula.*
* British Patent No. 470,432.