The measuring of the intensity of light given off by the burning of pyrotechinic pieces designed for signaling and illumination is an important factor and deserves careful consideration. Consequently a chapter on photometry is a necessary part of this work. The light given off by the burning of a signal must be of such an intensity as to make it visible from a distance prescribed by military specifications; also, the degree of illumination where pyrotechnic pieces are used for this purpose must be of such intensity as to throw a light over a definite area from a given height for which specifications are prescribed. It is necessary that there be devised means of testing for both signaling and illuminating articles that shall insure their conforming to specifications.
Fig. 211. - Standard photometer used in the Government testing laboratory.
Intensity of light must be measured by means of its quantity of illumination upon a given surface and a comparison of this illumination made with that of a light source having a known value. Thus a photometer is an apparatus by which we may compare the luminous intensities of two given sources of light. The fundamental principle of the photometer depends upon throwing the illumination produced from two different sources upon a given surface and adjusting the distance between the source of light and the screen until the degree of illumination shown by each is equal.
This fundamental principle, which is the basis of every photometer, is an immediate consequence of the fact that the eye appreciates with maximum precision the equality of the illumination of two surfaces, while the precision with which this organ can determine the ratio of two illuminations is illusory.
As a means of making a comparison and giving valuation to an unknown source of light, some definite value must be fixed upon for the Known source of light. The candle, owing to its common use at a time when photometric investigations were at their inception, was first taken as a standard, and though its proportions have varied in the course of time, it remains to-day as the unit for comparison.
The second fundamental feature to be considered is that the light rays emanating from a given source radiate in all directions, though each ray is promulgated in a straight line, in consequence of which it will be seen that the quantity of illumination falling upon a surface of given area will be in some ratio with its distance from the source of the light. Furthermore, it is evident that if the light is at the center of a hollow globe the total illumination will fall upon the inner surface and will be constant, no matter what the size of the sphere. Consequently, as the area of a sphere is in proportion to the square of its radius, the quantity of light upon a unit area must vary inversely as the square of the radius.
The unit distance at which to observe the illuminating power of a candle has been taken as 1 foot, hence our unit is the foot-candle. When moved to any other distance the proportionate illumination becomes 1/d2 where I is the unit or multiple of the unit, depending upon the number of candles to which it is equivalent, and d is the distance, in feet, of the illuminated surface from the light source. This value is the fundamental one for all determinations.
The specifications of the standard English candle are as follows:
In external dimensions it shall be 10 inches long, with 0.9 inch diameter at its bottom and tapered to 0.8 inch at its top.
The wick contains 3 strands of cotton plaited together, each strand consisting of 18 threads. The strands should be plaited with such closeness that, when the wick is laid upon a rule and extended by a pull just sufficient to straighten it, the number of plaits in 4 inches shall not exceed 34 nor be less than 32.
The wicks should be steeped in a liquid made by dissolving 1 ounce of crystallized boracic acid in a gallon of distilled water, to which 2 ounces of liquid ammonia have been added.
The wicks are then to be pressed until most of the liquid has been removed and to be dried at a moderate heat. Twelve inches of the wick thus made should not weigh more than 6.5, nor less than 6, grains. Ten untreated wicks when burned should have a weight of ash of not more than 0.025 grain.
The spermaceti of which the candles are made should be extracted from crude sperm oil, and should be so refined that it has a melting point lying between 112° and 115° F. Since the candles made from the spermaceti alone are brittle, and the cup which they form in burning has an uneven edge, it is necessary to add a small portion of beeswax or paraffin to remedy these defects. The best air-bleached beeswax, melting at about 199° F., is to be added to the spermaceti in proportions of not less than 3 and not more than 4 1/2 per cent.
Candles should weigh about 1/6 of a pound and the tension on the wick during molding should be about 24 ounces. The rate of consumption on burning should be 120 grains per hour. The height of the flame now accepted is 1.77 inches. The wick must be observed carefully and snuffed to maintain this height.
The standard candle has several disadvantages. The candlemakers have never been able to meet the specifications with a degree of accuracy to insure a uniform flame. The open flame is sensitive to air currents and the height of the flame difficult to maintain constant, due to a certain irregularity of cupping and the attention and skill called for in snuffing. At the present time the unit is so small as to add further difficulties in comparison with the size of the units to be measured.