Beyond Visible
Dealing with the Infrared Contamination of Ultraviolet Images

Shane Elen

What is Infrared Contamination of Ultraviolet Images
A reflected ultraviolet image should exhibit contrast resulting only from ultraviolet light and structural effects. Contrast resulting from other sources is undesirable as it can attenuate or mask the ultraviolet contrast. The presence of infrared light in an ultraviolet image acts in this manner and therefore needs to be eliminated or significantly reduced. The cause is often two fold, the camera CCD and the ultraviolet transmission/visible blocking filter attached to the lens.

Infrared free, ultraviolet image

Infrared contaminated, ultraviolet image

Infrared image

Reflectance spectra of the outer and central regions of the common dandelion. This shows that these two regions exhibit similar visible and infrared reflectance, but dissimilar ultraviolet reflectance.

Fig.1. Visible image of a dandelion taken by natural window lighting

Fig.2. Infrared image of a dandelion taken through a Tiffen 18A ultraviolet filter using a UV modified Nikon SB-14 but fitted with a custom wavelength SW5-IR infrared adapter (700nm cut-off) i.e. infrared light only

In order to control the lighting environment, the dandelion images portrayed here were shot indoors in the dark using a Nikon D70, 105 UV-Nikkor and a UV enhanced Nikon SB-14. The focus was set once for ultraviolet (figure 12) and not adjusted for subsequent shots, regardless of filter stack thickness etc.. All images were captured at 800ASA, f5.6 except figure 12 which was set to f8 to prevent overexposure. These images are not meant to be artistic but were shot only for demonstration purposes. Filters attached to the lens or the flash are indicated on each image. Other than noise correction and a little sharpening, they are basically as shot.

Fig.3. Kodak 18A Ultraviolet Transmission Filter (14% IR leakage)

Fig.4. Hoya U-330 Ultraviolet Transmission Filter (50% IR leakage but it does transmits SWUV)

Fig.5. Transmission characteristics of the Nikon SW-5UV Ultraviolet flash adapter exhibiting both ultraviolet and infrared

Fig.6. Ultraviolet image exhibiting a slightly washed out ultraviolet pattern, captured with Tiffen 18A and a UV enhanced Nikon SB-14 fitted with a Nikon SW-5UV.

Fig.7. The transmission spectra tell the tale - the Tiffen Hot Mirror does not prevent the infrared below 750nm from passing through the Tiffen 18A but it does prevent most of the ultraviolet! UNACCEPTABLE SOLUTION but can be salvaged by using Photoshop Channels (see below)

Fig.8. Ultraviolet image exhibiting a severely washed out ultraviolet pattern, captured with Tiffen 18A, Tiffen Hot Mirror and a UV enhanced Nikon SB-14 fitted with a Nikon SW-5UV. This is actually worse than just using the Tiffen 18A by itself because the Hot Mirror passes very little ultraviolet while freely transmitting infrared.

In the example below I have substituted a BG38 in place of the BG40 as I have misplaced the latter, however they are very similar in their characteristics.

Fig.9. Transmission characteristics of the Tiffen 18A and Schott BG 38 exhibit very little possibility of infrared leakage while maintaining high ultraviolet transmission when used in combination

Fig.10. Ultraviolet image exhibiting a stronger ultraviolet pattern, captured with Tiffen 18A, BG38 and a UV enhanced Nikon SB-14 fitted with a Nikon SW-5UV. Clearly an improvement over the sole use of the Tiffen 18A.

Fig.11. Transmission characteristics of the Baader Venus Filter exhibit a high ultraviolet transmission and only 0.5% infrared light leakage.

Fig.12. Ultraviolet image exhibiting a strong ultraviolet pattern, captured with a Baader Venus Filter and a UV enhanced Nikon SB-14 fitted with a Nikon SW-5UV. A smaller aperature was required to prevent over exposure but this still resulted in a brighter image as well as an increased depth of field.

Fig.13. Tiffen 18A & Tiffen Hot Mirror Combination

Fig.14. Hoya U-330 & Tiffen Hot Mirror Combination

Fig.15. Tiffen 18A & Schott BG38 Combination

Fig.16. Hoya U-330 & Schott BG38 Combination

The Tiffen 18A and Schott BG38 combination shows an infrared free spectra however the ultraviolet transmission of this combination has dropped from 60% (fig.7) down to 40%. The U-330 filter used in combination with the Schott BG38 results in a strong ultraviolet transmission around 60% with only minimal infrared leakage (<0.5%), minimal blue leakage around 460nm (0.5%) and a small region from 400-420nm (8% @ 400nm). With a few minor trade offs, these filter combinations compare favorably with the Baader Venus filter with regards to purity of ultraviolet light however neither come close to the 75% transmission exhibited by the Venus filter.

The ultraviolet image of the dandelion captured with an 18A filter in combination with a Tiffen hot mirror (fig. 8, 17 and 13) is a good case for using the blue channel data only. In this image, the minimal amount of ultraviolet occuring in the region below 355nm will occur primarily in the red channel along with much of the infrared from the 700-765nm region, so this channel can be considered unusable. The red channel can therefore be excluded (Photoshop) without significant loss of ultraviolet data. The region 370-400nm provides useful data in both the blue and the green channel. However infrared contamination occurs primarily in the red channel, followed by the green channel, and lastly the blue channel. Therefore, in this case, the blue channel is likely to exhibit better ultraviolet contrast than the green channel. Using only the blue channel (fig. 18) and excluding the red channel (infrared) and the green channel (infrared, ultraviolet and noise), permits the ultraviolet contrast to be somewhat salvaged from the original infrared contaminated image.

Fig.17. Ultraviolet image exhibiting a severely washed out ultraviolet pattern, captured with Tiffen 18A, Tiffen Hot Mirror and a UV enhanced Nikon SB-14 fitted with a Nikon SW-5UV. This is actually worse than just using the Tiffen 18A by itself because the Hot Mirror passes very little ultraviolet while freely transmitting infrared.

Fig.18. Ultraviolet image extracted from fig. 17 by using only the blue channel data in Photoshop and excluding the green and red channel. Although some noise and loss of detail has occured the overall result is not bad considering the small amount of UV light actually passed by this filter combination.

Fig.19. Red channel

Fig.20. Green channel

Fig.21. Blue channel

In the image of the dandelion captured with the 18A in combination with the BG38 filter (fig. 10 & 22), the ultraviolet transmission peak occurs around 370nm and so the green channel is likely to contain a lot of ultraviolet data whereas the blue channel will only contain a minimal amount. However, the filter/camera combination permits ultraviolet transmission down to 340nm so it is likely that the red channel will contain significant ultraviolet data in addition to the few percent of infrared that is passed by this filter combination. These predictions can be confirmed by observing the ultraviolet contrast exhibited by the dandelion in the red channel (fig. 23), blue channel (fig.24) and green channel (fig.25). In this case, excluding the red channel will result in significant ultraviolet contrast loss and would not be recommended.

Fig.22. Ultraviolet image exhibiting a stronger ultraviolet pattern, captured with Tiffen 18A, BG38 and a UV enhanced Nikon SB-14 fitted with a Nikon SW-5UV.

Fig.23. The red channel data from fig. 22 shows a reasonably good ultraviolet pattern in the flower indicating that it is making a significant contribution to the ultraviolet image and shouldn't be excluded.

Fig.24. The blue channel data from fig. 22 shows a noisy weak ultraviolet pattern in the flower.

Fig.25. The green channel data from fig. 22 shows a reasonably good ultraviolet pattern in the flower.

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© Shane Elen 2006. Last updated Jul 27th, 2007.