I good friend is an optical engineer and some of his passion for optics has rubbed off on me. Recently I've been studying the Camera Obscura and as part of that trying to learn about the lenses they use (Wiki: History of photographic lens design, Photographic lens designs). But I suspect I'll be adding patents for other types of things.
|The generic name:
Geneva Lens Measure
is on the dial along with the
Chicago Dial Indicator Co.
The hollow sheet metal cap protects the points when the meter is not in use.
When the points are placed on a flat surface,
such as a 123 block or V Block, the needle turns about 3/4 of a circle and points to 0.
So the position of the needle in the photo shows the maximum negative reading it can make of just over -19 dioptre. In a similar manner when the central pin is pressed the maximum indication is about +22 dioptre.
The separation of the outer points is 20.61mm
It turns out that in order to get the correct dioptre (Wiki) both the curvature of the lens and the refractive index of the lens material need to be known. The Lens Measure actually measures the radius of curvature (positive or negative) and has a built in adjustment for the refractive index of the glass. So when measuring plastic reading glasses or a magnifying glass the displayed result is wrong.
Magnifying glass (Wiki) FL = 6.75" (0.171 meters = 5.8 dioptre). Reads +3.2 and +3.2 dioptre (sum = 6.4 off by 0.6).
I don't know if the range of adjustment in the Lens Measure is large enough to include the refractive index of plastic.
The Wiki Lens Clock web page explains how to convert
the reading when the refractive index (Wiki) is different from the factory set value of 1.523 (crown flint glass).
|425 = 1
Measure the positive or negative curvature of a lens in dioptre.
R = (W2 + 4 * H2 / (8* H)
R is radius
W = horizontal distance between outer 2 points
H = + or - height difference between center point and outer points
The Radius will be given in the same units as W and H. To get dioptre the radius should be measured in meters, then Dioptre = 1/R (meters)
The power of a lens is equal to (n-1) * R where n is the refractive index (see link above for dioptre and the formula in the curvature section).
703725 Lens-measuring instrument, Franklin Hardinge (famous lathe maker), Jul 1, 1902, 33/507 - dial gauge type
1093307 Spherometer, Joseph Becker, Apr 14, 1914, 33/501 -
1151635 Lens-measure, Austin T Webb, Aug 31, 1915, 33/507 - dial gauge type
Curvature-measuring device,Bugbee Jr Lucian W, Continental Optical Corp, Aug 2, 1927, 33/507, 116/299 - looks like micrometer with 3-point end
Measuring instruments,Price George B, Oct 14, 1958, 33/838, 33/709, 33/555.3 -
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