DARKROOMS AND DARKROOM PRACTICE 565 



for the purpose of flushing the cement with a hose. The wooden overcovering will be 

 easier to stand and to walk upon than cement. There is the additional advantage that 

 beakers or graduates will not break so easily if dropped upon wood as if dropped upon 

 cement. Linoleum too is easier to stand and walk upon than cement. A layer of 

 air between the cement and the wooden decking will keep the feet warmer if the dark- 

 room tends to be cold. 



Darkrooin Eqmpmeiit. Safe Lights. — There is no truly "safe" light. Lights in 

 the darkroom are only relatively safe. The commercial devices known as safe lights 

 are safe in that materials viewed under them do not fog provided they are not held too 

 close to the source of illumination nor for too long a period. The "safeness" of such 

 sources of illumination depends upon their intensity and upon the color of light 

 emitted. Ordinary blue-sensitive films or papers may be viewed under a generous 

 amount of red or orange illumination; the more sensitive materials, however, must be 

 handled with greater caution. They must not be exposed to red or orange light for too 

 long a period. The more sensitive the material, the less exposure to the safe light will 

 be required to produce fog. 



Orthochromatic materials should be processed under a red light which is quite 

 dim if the materials are highly sensitive (fast films or plates). Panchromatic materials 



c: 



■Ve/our black normal 



I /i I I I I 1 1 ns«^ I 1 1 \i 



400 500 600 700 



Woivelengifh in Millimicrons 



Fig. 3. — Transmission characteristic of OA and O filters and response of Velour Black paper. 



are sensitive to all colors and must be processed in darkness or with the aid of a very 

 dim green light. The reason for choosing green lies in the fact that the eye is more 

 sensitive to green than to red and that this difference in favor of green increases as the 

 intensity of illumination is decreased. Orthochromatic films are verj^ sensitive to 

 green, but not to red, and for this reason a panchromatic green safe light should not be 

 used in processing orthochromatic materials. 



If a 25-watt lamp is used in a Wratten safe light or a 10-watt lamp in an Eastman 

 safe light, no fog should be produced on the material for which the safe-light filter is 

 designed, provided the material is exposed no closer than 3 ft. for a period of 30 sec. 



Red bulbs, which are cheaper than safe-light filters, are rarely safe. They must be 

 used with considerable caution to avoid fogged film or paper. 



An electric coil heater emitting very little light will fog bromide paper if held too 

 close to a tray in which the paper is being developed. 



The criterion of the best safe light for a given sensitive material involves the spec- 

 tral sensitivity of the material, the spectral transmission of the safe light, and the 

 physiological characteristics of the eye. The following data taken from the Defender 

 Trade Bulletin, September-October, 1938, are concerned with Defender Velour Black, 

 a projection paper. 



The question may be asked, which is better for this paper, a Wratten OA (greenish 

 yellow) or a Wratten O (bright orange), with lamps of equal wattage behind them. 

 The series O is much brighter than the OA and produces more fog. If, however, the 

 distances are adjusted so that equal illumination is secured on the paper, the OA is 

 less safe. This may be proved from the spectral characteristics of the safe light and 

 the paper. Note that in the OA the far red is eliminated but that the safe light 

 transmits in this region. 



