Device Cor Projecting an Image of a Reading Scale CLIFTON TUTTLE AND F. M. BROWN, Kodak Research Laboratories,
SINCE
the optical system now in use in these laboratories to facilitate the reading of microchemical balances has proved satisfactory, the authors believe a description might be of value to others engaged in similar work. For routine analyses, a number of Kuhlmann microchemical balances are used, in which the pointer passes across a white scale with black line indexes at 0.2-mm. intervals. Deflection of the pointer is estimated to a tenth of one of these divisions-an operation obviously impossible for the unaided eye. Ordinarily a monocular telescope magnifier giving 5 X or 6X magnification is directed at the scale. Operators who make many daily readings in this manner suffer considerable eyestrain and fatigue in making precise observations and would therefore find desirable a projected scale image that could easily be observed with both eyes. I t appears impossible to provide sufficient light on the conventional white scale to make possible its projection with adequate brightness and sufficient magnification. A light source of wattage high enough to provide sufficient light cannot be used inside or even in proximity to the balance case because of the danger of setting up convection air currents. The authors have solved the problem by replacing the white ivory scale with a highly efficient specular reflecting surface. For this purpose they selected a piece of optically polished stainless steel and upon its surface reproduced a replica of the microbalance scale by ineans of a technique worked out for the photographic reproduction of reticles. Mechanical engraving of the lines and subsequent filling in with black pigment would serve as well, but would probably be more expensive. The steel mirror surface reflects about 80% of the incident light, most of which is utilized, instead of scattering it inefficiently as does the matte white scale. The optical system actually employed in directing light to the scale and receiving it for projecting a magnified scale image might well be of various forms and dimensions, depending upon the particular apparatus to which it is applied. The system used in the microbalances is shown diagrammatically in Figures 1 and 2.
Rochester,
N. Y.
I n use, the balances are set up inside glass cabinets that protect the instruments from air currents and mechanical injury. The lamp in a housing is placed outside this outer cabinet. The filament of the lamp is imaged a t about 5 X magnification slightly above the stainless steel mirror. This slightly out-of-focus filament image produces acceptably uniform illumination on the mirror. The scale reflects the image into a 2.5-cm. (1-inch) focal length objective which projects the scale image and the image of the shadow of the pointer onto a white screen a t a magnification of about 8 X . For convenience, the image is directed downward by means of a small plane mirror, so that it falls on the bench in front of the instrument in a position which can be conveniently seen by the operator. The image is bright enough to be clearly visible in a well-lighted room. The illumination level is about 20 foot-candles.
ILLUMINATED SCALE
Figure 2.
\\\-
Side View of Pointer %de, and Projection Objective
Since it is important to avoid heat within the balance case, it is desirable to determine how much energy is absorbed inside the case. The energy radiated by the lamp and accepted by the first lens amounts to about 0.2 watt, 80% of which is absorbed by the heat-absorbing glass (Corning Aklo). About 0.04 watt is the amount of energy reaching the balance scale. Almost 30% of this energy is reflected by the mirror surface through the lens and about 70%, or less than 0.03 watt, goes to heat the instrument. This is less than the radiant energy entering the case from other light sources in the room. Prolonged trial shows no heating effect upon the delicate mechanism of the balances.
SPECTACLE
COMWJNICATION 979 from the Kodak Researoh Laboratories.
Spectrographic Boron Steel Standards T h e National Bureau of Standards, Washington, D. C., is prepared to furnish six samples of boron steels in rod form for spectrographic standards. T h e standard samples are cylindrical rods 7/32 inch in diameter and 4 inches long. T h e 4-inch rod may be c u t a t the center, giving two rods each 2 inches long for use as self-electrodes. T h e price per sample is $3.00.
1” ANASTIGMAT
,MOUNTED ON
FRONT DOOR
I
/
871,eX L
SCALE ON POL~SWED STAINLESS STEEL
Figure 1.
No.
OF IMAGE POINTER
AND
425 420 427 428 429 430
SCALE
Microbalance with Scale Projecting System
645
Kind
Total Boron. % 0.0006 0.0011 0,0027 0.0059 0.0091
0.019
