Light-emitting diodes are perfectly suitable for dark-room light, because they (a) obviate the need of filters; (b) emit cold light; (c) have a life that is not shortened by continuous on-off switching; and (d) do not radiate infrarays. The types used must, of course, have a high light output; fortunately, there are nowadays LEDs with a luminous intensity of hundreds of millicandela.
Circuit diagram :
The sensitivity of photographic paper lies between wavelengths 300 nm and about 550 nm, whereas the wavelength of the light emitted by green LEDs is about 565 nm; that by amber types around 585 nm; and that by red LEDs about 640 nm. From this, it is clear that all three types of LED may be used with impunity. None the less, in practice, it is best not to use green ones. Because of the special composition and high sensitivity of colour negative paper, only yellow LEDs with reduced light output should be used when processing this paper. The proposed light, therefore, has provision for reducing the emitted light. Note that since colour reversal paper is sensitive to all colours, it can only be processed in total darkness. When working with orthochromatic paper, only red LEDs should be used. With reference to the diagram, each group of three LEDs is fed from a current source, Ti to T6 respectively. The current level, and consequently the light output of the LEDs, is determined by the setting of Pti Zener diode D,9 provides the reference voltage for the current sources, ensuring that the light out-put of the lighting unit remains virtually constant over the life of the PP3 battery.
Maximum light output is set with the aid of P2. To this end, both Pi and P2 are first set to maximum resistance; after this, P2 is adjusted until a potential of 0.2 V is measured at point A. The maximum current through the LEDs is then about 20 mA. As the photograph shows, the unit has been constructed so that Si is easily operated. Since this switch is a press-to-make type, the light will switch off as soon as it is put aside, thus preserving the battery. It is possible to have the light on continuously by connecting an external battery to Uext. In that case, Rio must be matched to this source according to Rio=(Uext-9)l0 [k4] but only if NiCd batteries are used. If standard cells are used, D2o and Rio must be omitted.
If a variety of photographic paper is processed, it may be useful to be able to switch between red and amber LEDs. For that purpose, each of the eighteen original yellow LEDs is duplicated by a red LED, shown in dashed lines. Switch S2 may be used to select the relevant bank of LEDs (red or yellow) as required for the specific application.