PHOTOGRAPHIC SENSITOMETRY 



157 



deep red or a blue light, the luminosity is small, as the eye is relatively insensitive to 

 these colors. If the energy is concentrated in the infrared or the ultraviolet portions 

 of the spectrum, the luminosity is zero, for the eye is not sensitive to these radiations. 



An exactly analogous situation exists with regard to photographic materials. 

 Corresponding to the visibility curve of the human eye, the sensitivity of a photo- 

 graphic material to radiation of a given wavelength is known as its photobility. The 

 net or over-all response of a photographic material exposed to heterochromatic light 

 is the product of the spectral-energy distribution of the light source and the photo- 

 bility; it is termed the "photicity" of the material. Photicity of photographic mate- 

 rials is analogous to the luminosity of the human eye (see Chap. X, Light Filters). 



Although by no means a rigorous treatment of the subject, the previous paragraphs 

 will indicate the necessity of maintaining known or standard conditions for precise 

 sensitometric work. It is also necessary to determine just what is a suitable or 

 appropriate spectral-energy distribution for the light source used in sensitometry. 



300 



400 



700 



800 



Fig. 3. 



500 600 



Wavelength in Millimicrons 



Relative spectral intensity of mean noon daylight at Washington, D. C. 



Selecting a Standard Light Source. — In establishing standards for sensitometry, 

 it is desirable that the standards be such as will give useful results in practice. Since 

 a very considerable portion of all photographic negatives are exposed with daylight 

 illumination, there are certain practical advantages in choosing for the sensitometric 

 light source one which will provide a spectral-energy distribution giving white light 

 similar to that of daylight. Because "daylight" varies considerably both according 

 to time and geographical location, it is more practical to specify the characteristics of 

 the white light of the sensitometric source in terms of the average daylight over a 

 long period of time. Measurements on mean daylight have been made over a number 

 of years; the curve for mean noon daylight at Washington, D. C, is shown in Fig. 3. 

 Once having established a suitable standard so far as regards spectral-energy dis- 

 tribution, the next step is to find a means of realizing this standard under controlled 

 conditions. 



None of the light sources which have been used produce a spectral-energy curve, 

 of themselves, sufficiently like that of mean noon daylight to be suitable for photo- 

 graphic sensitometry. The most suitable, convenient, and precise light sources 

 (incandescent lamps) produce spectral-energy curves which are lacking in blue and 

 excessive in red radiations. However, the spectral-energy distribution of the tung- 

 sten lamp can be made to approximate that of mean noon daylight quite satisfactorily 

 through the use of appropriate (Davis-Gibson) filters. 



