Questions about studio lighting

[Manfred M]

Would a CTB gel prevent harm from UV radiation?

Likely. One would have to see the absorption curve of the filter and the emission curve of the light source.

[Manfred M]

Manfred....Seriously!!!

Quite seriously, Rob. The light shattering and the UV emissions are the reason that the shields are fitted on halogen light sources. Some bulbs do have integrated shields.

[xpatUSA]

The light unit arrived unexpectedly today, as I wasn't expecting it until next week. Oddly, it came with a used bulb that blew the second time I turned the unit on even though I seem to remember that the product was advertised as not including the bulb.

Sounds to me like item had been returned to the Vendor for some reason.

The light shattering and the UV emissions are the reason that the shields are fitted on halogen light sources.

I was about to question that but I've been burned a time or two this week on DPR so, for a change, I checked and found:

"Any unfiltered optical source, whose emissions are due to the heating of a material e.g. a filament lamp, that emits significant quantities of UV will also emit visible and infrared radiations. In the case of high temperature tungsten halogen lamps biologically significant amounts of shorter wavelength UVB are also emitted." (my bold)

At 3000K, that's 760 W/m^2 (Watts per square meter of filament surface), for what it's worth. At 3500K, that's 8050 W/m^2.

It also says:

"In applications where greater power (up to 5 kW) is required, tungsten-halogen (often called quartz-halogen) lamps are often used. <blah> The majority of the bulbs of tungsten-halogen lamps are made from silica (quartz) whose thermal properties are most suitable. The combination of filament temperatures which are likely to be in the range 2900 to 3450 K and quartz bulbs results in a significantly higher level of emission of potentially harmful UV compared with ordinary tungsten filament lamps. The incorporation of suitable secondary filtration to reduce UV emissions to an acceptable level is an important feature of the design of any illumination system using tungsten halogen lamps (McKinlay et al., 1989)." (my bold).

Quartz glass passes UV well, ordinary glass does not. Long ago, I was responsible for the design of a gas turbine flame monitoring system which used UV sensors and the glass window was in fact quartz.

Source:

http://www.inchem.org/documents/ehc/ehc/ehc160.htm
.

[Dave Humphries]

Naked Quartz 'glass' envelopes are a PITA to handle - you must not get skin oil on them (or anything else). Since this often creates a localised temperature gradient which may over-stress the Quartz envelope and cause it to shatter in use or fail prematurely - i.e. when it has (later) heated up.

This risk of accidental contact with the Quartz envelope (even when off) may quite high; e.g. from moving the lamp about when cool, splashes of red wine when doing wine drop photography - sorry Mike, I couldn't resist, I know you wouldn't waste good wine like that.


These days, many consumer Halogen lamps, even the ones of 20 - 50W are supplied with a a secondary glass envelope; as a globe or incorporating a reflector, to avoid all these issues. Including UV exposure from the beam.

I have not determined whether the lamp Mike is using takes a bi-pin/bi-wire variety, or if it is one of the 'linear' varieties about 3.5 inches long with a cap at each end, given the wattage, it may well be the latter. These are designed to go in fittings that usually do have the safety glass and be handled by what I'll term 'semi-professionals' who know how to handle them (been trained) and appreciate the dangers. Most of 'semi-professionals' wouldn't use one without the safety glass, particularly if they have a secondary safety responsibility to others (e.g. customers or colleagues).

However, just as most of us wouldn't take to a public road and drive a car without wearing a seatbelt or ride a motorcycle without a helmet - or shoot a lens without (normally) having a filter over the front element of their expensive lens - there will always be some people that don't see the need.

Please let's not go off topic with examples where all my examples are flouted daily by millions.

Thanks in advance on my last request, Dave

[Mike Buckley]

Sounds to me like item had been returned to the Vendor for some reason.

You probably overlooked that I bought it used from a reliable third-party vendor, in this case Roberts New & Used. They don't sell this model new by itself or as part of a set, so it wouldn't have been returned per se.

moving the lamp about when cool, splashes of red wine when doing wine drop photography

I don't understand the concern. There was no warning information included with the product indicating that it's bad to splash the lamp with wine of any color, much less red wine.

[Manfred M]

I don't understand the concern. There was no warning information included with the product indicating that it's bad to splash the lamp with wine of any color, much less red wine.

I don't know, Mike. You've been known to be a bit rough with your wine glasses and we wouldn't want you to have any problems with your new light.

[chauncey]

Just saying is all...http://www.bigasslight.com/homes-shops/

[xpatUSA]

You probably overlooked that I bought it used from a reliable third-party vendor, in this case Roberts New & Used. They don't sell this model new by itself or as part of a set, so it wouldn't have been returned per se.

oops

[Inkanyezi]

I don't know whether I am throwing a spanner in the works or if it isn't relevant to the task at hand, but I think it is worth the while to know that "daylight" spectrum is very different from black body radiation, and a gel that raises the colour temperature of a 3500 K bulb to 5500 K will present a totally different spectral curve to the scene compared to any daylight lamp.

The reason is that there are two different takes on the spectrum depending on K value. Below 5000 K, the "black body" spectral curve is used, and from 5000 K and above it is the "daylight" curve. Hence, if the blue filter that converts the incandescent bulb is not parametric, but only filtering out red, you cannot reach 5500 K with the correct spectral curve. It will be different from a daylight lamp. Its CRI at 3500 K will be almost 100, but when turned bluish with a gel to 5500 K, its CRI will go down to below 80. It might be of no importance, as it might not even be noticed in the actual scene, but the lamp will not be turned "daylight" if it is not filtered through a parametric filter, as for example neodymium oxide.

[Mike Buckley]

Thanks, Urban, as I have never heard of that distinction, not being technically minded. My guess is that the difference you mentioned will be of no practical importance due to the type of photography that I plan to do with the lamp. Even so, I'll look for any issues just to be safe.

[Mike Buckley]

Roberts New & Used is the company that sold me the used lighting unit. They have given me a $10 credit for the missing safety glass. This is an example of why the company has such a good reputation.

I ordered a new piece from B&H so I can tell my wife that I'm being super safe.

[Manfred M]

Makes perfect sense, Urban. A filter removes certain frequencies of light; if the frequency isn't there, there is no way of adding it.

I also agree, at least for a tungsten light source, the negatives you list are really more theoretical than practical. Photographers and movie makers have been fooling us with CTB and CTO gels for decades.

[Mike Buckley]

Photographers and movie makers have been fooling us with CTB and CTO gels for decades.

I've been fooling my wife for over three decades despite that I never had a CTB or CTO gel until last week. Now that I've got two CTB gels, I should be able to fool her at least another day or two.

[Inkanyezi]

Makes perfect sense, Urban. A filter removes certain frequencies of light; if the frequency isn't there, there is no way of adding it.

I also agree, at least for a tungsten light source, the negatives you list are really more theoretical than practical. Photographers and movie makers have been fooling us with CTB and CTO gels for decades.

I don't think we should regard them as "negatives", I only mentioned that it can be worthwhile to know that there are differences. Sometimes they are of no importance at all, maybe mostly, but sometimes they can be relevant, perhaps highly relevant. I believe that it is a Good Thing to know about these differences, as we might otherwise not understand why it does not turn out as we wish, in those cases where it might be important. So these differences can indeed be regarded as theoretical, but they are derived from practical experiments, and in some cases practical problems might arise from the colour differences that arise from different spectral distributions. In some professions they might even be of utter importance, as for example among dental technicians or car painters.

It boils down to the tri-stimuli representation of colour, which we could regard as metameric match. When colours match both under incandescent light and daylight, they will also match with an incandescent lamp that has been adjusted with a bluish filter to a higher K value, as well as with incandescent light filtered with a neodymium filter. There are few colours that can match under those conditions.

There are few instances where metameric match is regarded as necessary, but sometimes it is important. For the photographer, knowledge about how colour is rendered is a Good Thing.