Led light for contact printing

[cardiomac]

If you want to make one time investment and save energy you need to go with led lights otherwise bulb is fine for you

The problem with LED's is that commonly available, high-brightness units do not put out much light in the UV range where Lodima is most sensitive. High-brightness UV LED's are available but they are very expensive and require eye protection. Near-UV LED's at 405nM are relatively inexpensive and work OK, at least they require no eye protection. I have used them in a 4x5 diffusion lamp head for enlarging on to Lodima. Using 8 LED's at full power (about 24 Watts total), it takes about a minute to expose a typical 4x5 negative to an 8x10 print. The advantages are that there is no heat so there is no warping of the negative, the emitted wavelength of light passes easily through the enlarging lens, having a single dominant wavelength may improve sharpness through the lens, and no eye protection is needed.

[polyglot]

With longer wavelength UV (the cheaper UV LEDs), you don't have the "visual focus" / "chemicals focus" offset problem as with shorter wavelengths. And less of it will be absorbed by the glass.

[Tin Can]

Is that good or bad?

With longer wavelength UV (the cheaper UV LEDs), you don't have the "visual focus" / "chemicals focus" offset problem as with shorter wavelengths. And less of it will be absorbed by the glass.

[polyglot]

The longer wavelengths will be easier to deal with. For a start when you focus the print under the enlarger using visible light, you won't need to apply a focus adjustment and hope that you got it right. And your exposures are likely to be shorter for a given LED power. There's a reason that Nikon sells special quartz UV-Nikkors: glass lenses soak most of it up.

Think of sitting behind a glass window. You don't get sunburnt because the glass absorbs all the UV-B but the longer wavelength UV-A still comes through and (say you're a truck driver with chronic sun exposure through windows) turns you wrinkly.

[cardiomac]

The longer wavelengths will be easier to deal with. For a start when you focus the print under the enlarger using visible light, you won't need to apply a focus adjustment and hope that you got it right. And your exposures are likely to be shorter for a given LED power. There's a reason that Nikon sells special quartz UV-Nikkors: glass lenses soak most of it up.

Think of sitting behind a glass window. You don't get sunburnt because the glass absorbs all the UV-B but the longer wavelength UV-A still comes through and (say you're a truck driver with chronic sun exposure through windows) turns you wrinkly.

Good explanation.