Zomei IR-filter. Transmission graphs

[ulf]

The five Zomei IR-filters was the first I got when trying my full spectrum converted camera.

I have measured the transmission as I could not find any published graphs and was curious if the were similar to Schott glass.

 

These are the results:

 

 

 

 

The measurements were done with an Ocean Optics USB-spectrometer and a very primitive halogen light-source.

The absolute amplitudes are not that accurate due to the ad hoc setup, but I think it is reasonable to trust the transition frequencies.

(When I eventually have a better setup and light source I hope to get more accurate results.)

 

It looks like Schott has materials that fit my curves rather well.

As Zomei seams to aim at reasonable quality they might even be using Schott glass in their filters.

[Andrea B.]

Ulf, thank you for the transmission charts. :D Eventually I try to port them over to the Transmission Chart area.

 

All optical filter glass in the world uses the same basic formulas. The Chinese filter companies are using Chinese-poured optical glass made according to these formulas. Zomei is not using Schott glass (or Hoya or Kopp or whomever). But, no matter, the Chinese filters typically can be sold cheaper. If the glass quality is the same and the transmission capability is the sam, then we all benefit from less expensive filter glass.

 

Some Chinese filter glass is excellent and some is not.

If you buy the less expensive filter glass, be sure to examine it for waves, striations, pits, gouges, non-bubble impurities, and any degradation in sharpness. If you can, look at whether the glass transmits as expected. If you get a bad filter, return it.

One example, a ZBW1 1.5mm UV-pass filter glass showed too much leak in the Visible area in this test: Testing for Visible Leak in the ZWB1 (1.5mm)

[OlDoinyo]

I have a Zomei 850--I have no real complaints about it. It seems to work OK. Some off-brand filters do have problems with image quality, however, even if the absorption curve is correct (Rocolax comes to mind.)

[Andrea B.]

Clark, what is the nature of the quality problem? Waviness? Or reduced sharpness? I am just curious.

[ulf]

I have a 77mm Rocolax 630nm filter and have not seen any problems with the images.

This might be because I have not looked carefully enough at the results.

 

I just checked the filter and cannot find any surface flaws or inclusions.

 

[OlDoinyo]

My problems with the Rocolax 1K seem to be reduced image sharpness and perhaps a slight veiling (the latter is not, admittedly, very significant.) It is possible that interactions of the filter with my lenses are to blame, or just focusing difficulties in general (the autofocus does not seem to work well with that filter.)

[ulf]

If the "Rocolax 1K" is a 1000nm IR longpass-filter I have a similar experience with the Zomei 950nm.

 

The transmission curve I measured for Zomei 950nm looks like the Schott RG1000 transmission with a less steep slope than the RG850 and Zomei 850nm.

 

I have stopped using the Zomei 950nm as the Zomei 850nm give me the same monochrome images with better quality with a much shorter exposure time.

 

I don't know the reason for the worse quality, but have a feeling that among other problems the hotspot problem is more pronounced with the Zomei 950nm.

The problem with hotspots vary with lens type, used aperture and motive.

I don't have any links at hand, but there are sites with lists of lenses and their level of hotspot problems.

 

It would be logical that lenses functioning less well the further away the light are from normal wavelengths they are designed for.