J. Spencer Love (jslove) wrote,
J. Spencer Love
jslove

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Well, that puts me in my place

I am working on the lighting plan for the studio control room (and, by extension, for the isolation booth and main recording room).

What kind of lights should I use?

Fluorescent lights buzz. Compact fluorescents buzz less, to be sure, but they also make noise. Incandescents are much quieter, but to make them really quiet, they should run on direct current. Still, unless I use a dimmer, even on AC they are so quiet I should not worry about it.

The problems with incandescent lights are
  • they generate heat
  • with dimmers, they become little sound transducers and can become quite loud at low light levels
Halogen lights are slightly more efficient, but
  • they still generate lots of heat
  • using dimmers with them is right out, since the tungsten cycle stops working
I've been looking into LED lighting, since it is more efficient than incandescent, and is quieter than fluorescent. LED emitters are beginning to approach the efficiency of fluorescents, but LEDs need current sources as their power supplies, not the more conventional voltage sources, and so the overall system efficiencies available about split the difference between the other two technologies. At least the hot, potentially humming power supplies can be located outside the studio.

The issue of LED dimmers is still up in the air. LEDs running on DC should be completely silent, but if they run on AC or are pulsed (which is the common dimmer technology and is also supposed to increase their lifetime), they may emit audible or electronic noise. So I am investigating that. The main problem with LEDs is their cost per lumen, which in terms of initial investment is much higher than either of the others, though total cost of ownership can be the lowest.

So I have been doing research, especially into LED systems. One of the front runners seemed to be the Xtreme Linear LED light fixture. At the specified luminosity and price, it looked like I could have adequate light for about $500, which is not too bad.

However, there seem to be some problems with the data sheet. My analysis showed that one possible explanation for the contradictions would cost me $5,000 for 6,000 lumens, far beyond my budget. So I wrote them the following (rather long) letter.

Subject:   Xtreme Linear Light
To:        Sales@theLEDlight.Com

Does this product exist?  If so, I am intensely interested in
using a number of them to light my sound-recording studio, but
I have a number of questions and problems with its data sheet.

Can these lamps be used as room lighting?  The 135-degree view
angle certainly gives this impression.  My control room has
approximately 15 square meters of floor space, so illuminating
this to an average level of 400 lux would require 6,000 lumens.
Since the walls would also be illuminated, additional lumens
would be required, but I don't require 400 lux uniformly
throughout the room.  According to the data sheet, using 5
XLLB2s would provide 6,400 lumens at a cost of $481.50, plus
taxes, shipping, and the cost of other system components.

These fixtures are clearly specified to operate at between 11
and 14 volts, DC, and to work with a (nominally) 12-volt
dimmer.  They are also specified to use small amounts of
current, but this can not also be correct:

[Table 1]

               mA   Watts   Lumens   Lm/W
   XLLB2      350     4.2    1,280    304
   XLLB3      400     4.8    1,680    350
   XLLB5      750     9.0    2,620    291
   XLLB6      800     9.6    3,180    331
   XLLB9    1,200    14.4    3,980    276

The power-dissipation is impressively low.  Too low.  White
light (one idealized type) is 242 lumens per radiated watt; all
of these numbers would require getting more energy out of the
fixture than is put in, i.e., efficiencies greater than 100%.
If the light need not be white, these numbers would be
remarkable but at least possible.

Either the current is wrong, or the lumens are wrong, or the
voltage is wrong.  My guess is that the lumens are wrong.  I'll
try to construct plausible values below.  It clearly states
that there are 58 lumens per LED.  Assuming that each emitter
is one LED, then we see

[Table 2]

           Lumens   Lm/W
   XLLB2      116     27
   XLLB3      174     36
   XLLB5      290     32
   XLLB6      348     36
   XLLB9      522     36

At that rate, I would need more than 100 emitters to get 6,000
lumens. At XLLB prices, that would be somewhere around $5,000.

The Lumens/Watt measured at the LED is probably better.  The
variation in Lumens/Watt works if you run series strings of 2
at one current (310 mA), and strings of 3 at another (400 mA);
this suggests that other circuitry (limiting current) inside
the fixture is dissipating some of the power; the actual LEDs
are plausibly producing about 50 Lumens/Watt, given the spec
of 58 Lumens and a typical dissipation of 1.16 Watts (350 mA at
3.3 volts typical, per the data sheet).

However, if the full current passes through the LED as I would
expect from series current-limiting, then the 400 mA strings
should be brighter, and so the spec of uniform luminance
suggests something else might be going on.  If the luminance is
uniform for all LEDs in all fixtures (neglecting variations in
individual LEDs from manufacturing), and assuming that no
switching power supplies (which would generate radio-frequency
noise) is used, I would expect the maximum current specs to be
as follows:

[Table 3]

              mA   Watts
   XLLB2     350     4.2
   XLLB3     350     4.2
   XLLB5     700     8.4
   XLLB6     700     8.4
   XLLB9   1,050    12.6

Note that the XLLB2 and XLLB3 would use the same amount of
power, but one third the power that the XLLB3 would emit as
light would be wasted as heat by the XLLB2's current regulator.
Similarly for the XLLB5 and XLLB6.

The Cree XLamp XR-E LED data sheet gives ratings in Lumens
rather higher than 58, but the values for the 7090-XR are not
given specifically, and one assumes the lower values for the
warm-white color.  Some additional loss may come from the
135-degree spec, assuming a diffuser is used.  I can't tell
from the product picture; it looks like three three yellow dots
may be bare LEDs, but if so, what is the translucent part of
the fixture for?

If the luminance ratings are other than the rather discouraging
ones in my Table 2, then the XLLB strips must use more than one
LED per emitter, i.e., they would be mounted in clusters.  I
can not say how likely this would be, since the individual LEDs
cost more than $10 in 100 quantity, but there are 6 square
inches of heat-sink per emitter in these fixtures.  To reach
the luminance specs, you'd need something like 10 LEDs per
emitter.

[Table 4]

              Spec    10x58
   XLLB2     1,280    1,160
   XLLB3     1,680    1,720
   XLLB5     2,620    2,900
   XLLB6     3,180    3,480
   XLLB9     3,980    5,220

A 100W incandescent light bulb typically radiates about 1,700
lumens.  (Yes, I know the incandescent may deliver fewer useful
lumens, depending on the fixture.)  This is comparable in light
output to the XLLB3 and is only twice as much power.  A compact
fluorescent with similar luminance uses only 23 watts, and is
therefore twice as efficient as the LED.  However, only about
35 watts of the XLLB3's (or 23 watts of the XLLB2's, i.e.,
1.15 watts typical per LED) output are actually used to emit
light.  The rest of the power is used by the current limiter.
For my purposes, it would help if I could put the current
source outside the studio, where waste heat is a particular
problem.

At the top of the Xtreme Linear Light web page, it says that I
should always use a dimmer, because its pulse-width-modulation
component should extend the life of the LEDs.  LED lighting
interests me because LEDs are completely silent, as DC-powered
incandescent bulbs would be, and unlike fluorescent or
AC-driven incandescent bulbs.

A dimmer module could cause the LED fixture to emit sound and
electromagnetic noise, and so the frequency of those pulses
becomes an important factor, and the wires must be shielded.
(At least the strobe effect would not be important in a
sound-recording studio.)  Is it really the case that these
fixtures sold without a dimmer are not warranted?  How does
the dimmer module affect the efficiency and lifetime of the
LEDs?  Can you point me at some reference material on this?

Could a DC dimmer be implemented in the current limiter
instead?  The brightness of the LED should vary with the
current.

It looks like reading color codes (e.g., on electronic
components) would benefit from adding some red and green light
to compensate for the white LED spectrum, which has a narrow
peak in the blue and a wider peak in the yellow, but is
definitely deficient in the red and green.  The diffusing
fixtures, however, are not available in any colors other than
white.  What would you recommend?

                                          -- Spencer


This was their reply:

From:      the LED Light
Subject:   Re: Xtreme Linear Light

Spencer, 

You are too much smarter than the engineers who designed the Xtremes.
Therefore, we cannot sell the product to you because you will not be happy
with them.

Regards,

The LED Light, Inc.
Ph 775-841-4490
Fax 775-841-4491
http://www.theledlight.com/index.html

That's what I get for writing such a long letter, I guess.
(I have redacted the personal name of the respondent.)
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