CHAPTER 10 MEASUREMENT SYSTEMS EMPLOYED IN OPTICS

TM 9-258

CHAPTER 10

MEASUREMENT SYSTEMS EMPLOYED IN OPTICS

The power of other lenses than those of 1-meter focal

10-1. Candlepower and Foot Candles.

length is the reciprocal of the focal length in meters and

a. The intensity of illumination on a surface

varies inversely as the focal length. This means that a

depends upon the brightness of the light source and the

converging lens with a focal length of 20 centimeters or

distance of the surface from the light source. The

1/5 meter has a power of +5 diopters, while a diverging

brightness of any light source is measured in units of

lens with a focal length of 50 centimeters or 1/2 meter

candlepower. One candlepower is the rate at which a

has a power of -2 diopters. The lens with the shortest

standard candle emits light. A standard candle was

focal length has the greatest plus or minus power in

initially a sperm whale oil candle, 7/8 inch in diameter,

diopters.

which burned at the uniform rate of 120 grains per hour.

c. Prism Diopter. A prism with a power of 1 prism

Current standards of candlepower are electric lamps

diopter will deviate light by 1 centimeter at a distance of 1

which may be obtained from the Bureau of Standards.

meter from the prism.

Thus, the prism diopter

b. The amount of illumination on any surface

designation of a refracting prism is the measurement of

depends upon the distance of the surface from the light

the amount it can bend light.

source. A candle 1 foot away from a screen will shed

10-3. Degree System.

more intense light on a given area of the screen than it

would if it were 3 feet from the screen. In the latter case,

a. The degree system is a means of measuring

it would illuminate a larger area with less intensity. The

and designating angles of arcs. The degree is 1/360 part

intensity of illumination is measured in foot candles. A

of a circle or the value of the angle formed by dividing a

foot candle is the amount of light intensity received on a

right angle into 90 equal parts. Each degree is divided

surface 1 foot away from a light source of 1

into 60 parts called seconds.

candlepower.

b. In the use of fire-control equipment, either

c. The intensity of illumination or the quantity of

degrees or mils (para 10-4) may be used to designate

light which is received per unit surface varies inversely

angles of elevation and azimuth. One degree equals

as the square of its distance from the source.

17.78 mils. One mil equals 0.0560 or 3 minutes 22 1/2

seconds. Refer to the conversion chart (fig 10-1) for the

Candlepower

approximate relative values of given angles of elevation

foot candles =

and azimuth in degrees and mils.

(distance)

10-4. Mil System.

a. The artillery mil system is a means of angular

d. The amount of light falling upon a surface is

measurement that lends itself to simple mental

measured in lumens. A lumen is the amount of light

arithmetic.

It provides accuracy within the limits

falling on an area of 1 square foot at 1 foot distance from

demanded by the military forces with distinct advantages

a standard candle (a above). Therefore, total lumens

of simplicity and convenience not afforded by any other

divided by area equals foot candles of illumination.

method of angular measurement. It is based on an

10-2. Diopters.

arbitrary unit of measurement known as the mil. The mil

a. General. The lens diopter or simply diopter is

is exactly 1/6400 of a complete circle.

the unit of measure of the refractive power of a lens or

b. The mil is very nearly the angle between two

lens system. The prism diopter is the unit of measure of

lines which will enclose a distance of 1 meter at a range

the refracting power of a prism. Both are based on the

of 1,000 meters (fig 10-1). Exact computation of the

metric system of measurement.

distance enclosed at 1,000 meters by 1 mil gives the

b. Lens Diopter. A lens with a focal length of 1

result of 0.982 meters. Therefore in assuming that 1 mil

meter is internationally recognized to have the power of 1

encloses 1 meter at 1,000 meters, an error of 0.018

diopter. The power of a converging lens is positive and

meter or 1.8 cm is introduced. This error is negligible for

that of a diverging lens is negative.

all practical purposes in the use of fire-control

instruments.

10-1