Best time of day to shoot UV outdoors?

[Doug A]

A bright sunny day usually has more UV at midday. I've been shooting during this time. Recently, a blog (not here) said to avoid this because IR is the strongest and has more chance to overload the blocking filter. Do you agree with this? Does the ratio of IR to UV change on a sunny day?

 

Thanks,

Doug A

[enricosavazzi]

It is often asserted in the scientific literature that the ratio of NUV / VIS is higher at dawn and dusk than during mid-day or night. However, this may not help in practice, because the absolute level of UV at dawn and dusk is generally too low to be usable with reasonable exposure times.

 

Whether it is better (in terms of ratio of NUV / NIR) to shoot at mid-day versus mid-morning or mid-afternoon is debatable, and in any case it is something you can evaluate by trying it for yourself. Depending on your equipment (especially UV-pass filters), the amount of leaked NIR may or may not be a problem. Solar irradiation and its NUV content also vary quite a bit with latitude, altitude and general weather, so it is difficult or impossible to state a general rule.

 

Unless you require an unaltered solar spectrum, you can also "help" the ambient UV, at least in close-up photography, by enhancing it with UV LEDs or a suitable electronic flash filtered with a UV-pass filter (or a NIR-cut filter) on the flash head. UV LEDs emit no significant NIR, and the abundant NIR radiation emitted by electronic flash tubes can be filtered out. Properly done, this also has the effect of increasing the NUV / NIR ratio.

[Andrea B.]

The attached chart shows that there's more IR light at 6am than there is at noon. (That blog source is wrong.) Makes sense because the longer IR wavelengths are less scattered when traveling thru early morning earth's atmosphere than are the shorter UV wavelengths. (I couldn't find a chart showing a later time.) But like Enrico says, location matters. If you are on a beach, for example, there is lots of light bounce so UV photos are not all that difficult in the not-too-early morning.

 

We talk a lot about IR leakage through our UV-pass filters, but the good ones block IR well enough that in practice it takes a lot or work to force enough IR through them so that a UV photo might get IR-contaminated -- with the exception of some ZBW glass. The general problem is not that there is a lot of IR, but that there isn't much UV.  

And again, as Enrico has pointed out, a bit of supplementary UV really helps even outdoors. I use my UV flash outdoors quite often unless making long view landscapes.

 

An interesting thing to note is that outdoors a broadband UV light source works better, in my opinion, than does a narrow-band UV-Led. Using a narrowband UV-Led on, say, a flower group outdoors seems to cause a blotchiness in the (eventual) false colors. It's like the UV-Led rather overwhelms the sunlight a bit. And it's hard to light-paint a short outdoor exposure.

 

 

[dabateman]

Small other factors might matter as well. The sun was well aligned for some of my flowers at 4pm.

However, traffic leaving a government complex was quite busy at that time leading to lots of car exhaust and thicker air. Our major street is typically jammed solid with cars.

But now many dive electric cars and its much better. So 3pm to 6pm isn't as bad as it once was. Its actually noticeable. 

I would say to just try and find the best angle of light for your photo or add it with a flash or flashlight. 

[Andrea B.]

(Side Note:  Happy to hear that many are driving electric cars. I'm thinking about getting one.)

[Kai]

Once I found data from Wales/UK (sunlight, clear sky):

https://www.schildi-online.eu/images/stories/uvb-fachartikel/UVBArtikelBild5.gif
 

[Doug A]

On 10/4/2021 at 1:35 PM, enricosavazzi said:

 

 

 

Whether it is better (in terms of ratio of NUV / NIR) to shoot at mid-day versus mid-morning or mid-afternoon is debatable, and in any case it is something you can evaluate by trying it for yourself. Depending on your equipment (especially UV-pass filters), the amount of leaked NIR may or may not be a problem. Solar irradiation and its NUV content also vary quite a bit with latitude, altitude and general weather, so it is difficult or impossible to state a general rule.

 

Unless you require an unaltered solar spectrum, you can also "help" the ambient UV, at least in close-up photography, by enhancing it with UV LEDs or a suitable electronic flash filtered with a UV-pass filter (or a NIR-cut filter) on the flash head. UV LEDs emit no significant NIR, and the abundant NIR radiation emitted by electronic flash tubes can be filtered out. Properly done, this also has the effect of increasing the NUV / NIR ratio.

Good points. Appreciate the insight. Hadn't considered adding UV. 

 

Thanks,

Doug A

[Doug A]

Thanks Andrea, Dabateman, and Kai for the charts and information.  Glad to know I wasn't making a huge mistake shooting at noon time. Living 500 ft. above sea level makes it tough to capture lots of UV. Gotta speed up the UV flash project. 

 

Thanks,

Doug A