| Description. | Temp. Deg. C. | Temp. Deg. F |
| Lowest red visible in darkness | 470 | 878 |
| ” ” ” daylight | 475 | 887 |
| Dull red | 550 to 625 | 1022 to 1157 |
| Full cherry | 700 | 1292 |
| Light red | 850 | 1562 |
| Full yellow | 950 to 1000 | 1742 to 1832 |
| Light yellow | 1050 | 1922 |
| White | 1150 | 2108 |
If it were possible for all observers to detect exactly the colours to which these temperatures refer, the table would be of great utility; but in practice any two persons might differ in judgment to the extent of 50° C. below a yellow; and when the white is reached, and becomes dazzling, accurate discrimination is impossible. At the same time, a trained workman, used to quenching steel at a fixed temperature, say 850° C., acquires a high degree of judgment with constant practice, and may not vary by more than 20° C. at temperatures below a light yellow. The personal equation, however, is too great for colour judgment by the unaided eye to be taken as an accurate guide to temperature. A fairly close approximation, however, may be obtained by matching the colours against prepared standards, as will be referred to later.
The determination of the intrinsic brightness of the heated substance by a photometric method naturally suggests itself as a possible means of ascertaining temperatures by optical means, and it will be found that all the optical pyrometers used for industrial purposes are based on this procedure. The law connecting the intensity of the whole of the light waves emitted with temperature, for a given solid, is approximately given by Rasch’s formula:—
| ┌ | ┐ x | |||
| I1 | │ T1 | │ | ||
| —— | = | │ —— | │ | |
| I2 | │ T2 | │ | ||
| └ | ┘ |
where I1 and I2 are the intensities corresponding to absolute temperatures T1 and T2; and the exponent
| 25000 | ||
| x | = | —— |
| T1 |
Hence at 1250° abs. the brightness increases as the 20th power, and at 2500° abs. as the 10th power of the temperature. This rapid increase in brightness for a small rise in temperature enables small increments to be readily observed; but a difficulty arises in practice owing to vast differences in brightness displayed by different substances at the same temperature. For example, the light emitted by an incandescent gas-mantle, which consists of thorium oxide, is vastly greater than that given out by a metal, such as platinum, at the same temperature; and it is therefore evident that the luminosity of a substance depends not merely upon its temperature, but also upon its nature. It is possible, however, to obtain indications for any substance in terms of a black body; thus if a heated solid possessed the same intrinsic brightness as a black body at a temperature of T, the “apparent” or “black-body” temperature of the solid would also be called T. All that this would signify would be that the condition of the solid was such that the light radiated was equal in intensity to that emitted by a black body at temperature T; and to obtain the true temperature of the solid, T must be multiplied by a factor which expresses the ratio of its emissive power to that of a black body.
In all photometric methods a standard light is employed, which should not vary in brightness, and with which the light from the source is compared. In optical pyrometers no attempt is made to measure the illumination in terms of candle-power; all that is necessary is to bring the standard and the source to the same degree of brightness by suitable adjustments. Amongst the standards employed are carbon-filament electric lamps, amyl-acetate lamps, and for higher temperatures the centre of an acetylene gas-flame; each of which is capable of producing a fixed degree of brightness when used under specified conditions. A black body, at known temperatures, is compared with the standard used, thus furnishing a scale of “black-body” temperatures to which the indications of a given source may be referred, as explained in the previous paragraph. Above 1000° C., however, the light becomes too dazzling to enable a proper comparison of the standard and source to be made, and absorbing glasses must then be used to reduce the brightness. Any coloured glass, taken at random, might not reduce the standard and source equally; but if a monochromatic glass be used—that is, a glass which transmits light of one wave-length only—a well-defined relation is found to exist between the intensity of the transmitted light and the temperature of the source. As optical pyrometers are used for temperatures above 1000° C. in most cases, involving the use of such glass, it will be necessary briefly to consider the relations between the wave-lengths of light and the temperature of the radiating substance, which in all cases will be assumed to be a black body.