Hi,

I have a Dazor desk lamp that requires two F15T8 tubes. I initially installed two 827 GE Polylux extra-warm tubes, but my girlfriend uses it to paint illuminated letters, so I recently switched to "full spectrum" 950 Philips Natural Sunshine tubes (that both of us love).

My girlfriend noticed that the Dazor's shade became warmer with the new tubes, especially on the sides.
So here's the question: is it possible that the generated heat depends on the phosphors? or just efficacy?
The Polylux is rated 1000 lumens while the Natural Sunshine is rated 590 lumens: I assume that the energy that is not converted into light is "lost" as heat, hence the higher shade temperature with the less effective tubes.

Does it make sense? Thanks for your help


(I didn't want to make experiments by switching the old and new tubes, because I have nothing to measure that heat and because the tubes are not easy to change on this Canadian model so I don't want to break them.)

All tubes are very inefficient, so most of the power is converted to heat. I think the difference from the phosphor is negligible and some other effect is at work :

Maybe the tubes are electrically different (within manufacturing tolerances), so it heat a bit more

Maybe the new tube emits more spectrum in a region which the shade sinks rather than reflects. so more of the light is converted to heat in the shade itself and less reflected

Interesting, I didn't think of that because both tubes are recent and come from well-known manufacturers.
As far as I understand, there is nothing surprising here and several factors may be involved.

The effect can be quite significant!  When making half coated F40T12's a few years ago I was surprised to notice that the portion of tube with no phosphor coating runs dramatically hotter than the portion equipped with phosphor.  This is logical.  When the UV energy from the discharge is absorbed by the glass, it is converted into heat.  In case one type of phosphor (e.g. a halophosphor or an inefficient deluxe colour) is less efficient at converting UV into visible light than another type (eg triphosphor), there will be more heat generation and the surface of the tube should indeed run hotter.

And another aspect related to the generated heat: Higher efficacy does not equal lower losses.
The reason is, than even lower efficacy lamps may still convert the UV into light with high energy efficiency (so there is no more power left to heat up the phosphor), but the generated light is on a wavelengths, where the eye is not sensitive. So ven when there are not much lumens, the radiated power could be still high.

This mismatch between efficiency and efficacy is very frequently ignored, frequently yielding wrong assumptions, mainly about related thermal load.

Medved: "Efficiency" and "Efficacy" are synonyms. Both words means efficiency in English or "יעילות" in Hebrew (At least how they appears on Google Translate). So why the distinction between these two words is important?
Or that I just asked the opposite of what you said?

In relation to light and illumination, there is strong difference. Don't forget many languages do not have really equivalent terms for the samne things, so vocabulary authors tend to use the nearest one they think is correct. But as these people are not experts, they could be frequently wrong (nobody could be an expert for everything, so it should not be seen as their fault, but you should always anticipate such kind of errors...)
E.g. in Czech language there is no really equivalent term for an "efficacy" either, even when many people very frequently use the incorrect "ucinnost" (what mean "efficiency")

"Efficiency" means a ratio of the output vs inputs expressed in exactly the same units, with exactly the same physical meaning.
That include obvious power transfer efficiency (W/W), energy efficiency (J/J), quantum efficiency (#particles/#particles) and so on. As it is a ratiometric measure, it is frequently presented in units like percents, ppm,...

The "Efficacy" mean a ratio of values of not directly comparable metric, like the lm/W.

So when speaking about light sources, one light source could have an efficiency of e.g. 20% (so waste 80% of the input power as heat), but an efficacy of 110lm/W, while another light source could have the efficiency 30% (so waste only 70% as heat) and efficacy of only 90lm/W.
So from these two, the one with higher efficacy (so generating more light) is the one with lower efficiency (so the output power is lower).

The luminous flux is not the same as radiated power. They are related via the spectral sensitivity curve and the spectrum of the light, but not the same. So when the first radiate only in the spectrum part just around where the eye is most sensitive, it generate a lot of lumens with only small radiated power, spo even when it's efficiency (radiated power / input power) is low, it's efficacy (luminous flux / input power) could be still high, compare to a source, what radiate a lot of power on wavelengths, where the eye is not as much sensitive (e.g. to get good color rendering)

Exactly this is the reason, why the full spectrum (CRI >90) fluorescents have lower efficacy than CRI80 tri-phosphor, even whenn they have about the same energy efficiency: The high efficacy tri-phosphors sacrifice some color rendering just to not spent power on wavelengths, where it does not contribute to the lumen output (mainly the red), while to reach the high color rendering quality you have to radiate across the complete visible spectrum, include the deep red, where the radiated power practically does not count for the lumen output.

And this is way more important with LED's, as there is much more freedom in spectrum design to optimize the color quality vs efficacy.
And it is the way, how some LED's could reach high efficacy figures without being extraordinarily energy efficient (that is, where the main technology advance happen).
So some cheaper maker could easily reach the efficacy figure of high-tech brand product: The cheaper one concentrate in his design all the radiated power into the spectrum, where the eye is most sensitive, so does not need as high energy efficiency to reach the desired efficacy figure.
The difference is, the real hi-tech brand reaches that efficacy with better color rendering...