Harman Direct Positive Spectral Sensitivity

[JAMESCF]

I’m interested using Harman direct positive paper in camera. As the paper is orthochromatic (I have included the spectral sensitivity graph) I was intending on experimenting with using red filters for portraits for a more flattering effect. (As I don't have any experience with the paper yet I don't know how necessary this is but I tend to like this look compared to orthochromatic rendering of skin tones.)

My question though regards the use of filters with orthochromatic mediums in principle. It makes sense that the spectral sensitivity can be adjusted by raising the relative sensitivity through use of filters, but what if the paper is not sensitive in the range that is filtered for, will this have an effect i.e. you can't increase from 0. For example, referring to the spectral sensitivity graph, what would the effect of a 650nm filter be given that the paper has no sensitivity at this wavelength? Therefore would a red filter be recommended?

Thanks,
James

 

[Terry Jones]

I believe that most red filters around the 650-670nm area are high pass filters so everything below that wavelength will be attenuated and all infrared wavelengths above will be passed. So in your case, a mostly blank piece of paper, assuming the filter is truly a high pass.

 

[Ian Grant]

First the recommended Safelight for Harman Direct Positive paper is a Dark Red 906 (Wratten 2) this is a very dense dark red filter. I was lucky enough to find a 10"x8" 906 filter NIB at a camera fair 3 or 4 years ago.

As the paper is Orthochromatic and slow filter at the taking stage filters would be counter-effective, as Terry has written a 650nm is effective;y a very dark red safelight filter.

Ian