I got my hands on an old slide projector (the kind that projects cel slides onto a wall).

The light source, is a 24V, 150W incandescent halogen lamp.

As a little pet project, I'm thinking of converting this projector to LED, using a high power LED. To do that, I need to remove the lamp mirrors, replace the existing lens with a reflector and collimator lens, add a heatsink, etc.

But before even attempting this, I need to know how much light I need.

I've used a conversion table to estimate the number of lumens that I need. My estimation is between 3.6klm to 5250lm (I've used "tungsten quartz halogen (12–24 V)" and "photographic and projection lamps", I don't really know which one is applicable in my case).

Only once I know how much light I need, I can deduce what kind of high-power LED I need.

  • I have no experience in photometry as such, is using Lumens as a comparison value even applicable?
  • Are my estimations correct?

I was looking at getting a 100W LED, which supposedly provides 10klm, and then add a PWM controller to dim it, if necessary. But it seems that a 100W LED is pretty overkill to me.

  • \$\begingroup\$ I am no expert but a 100W LED seems an overkill to me too. \$\endgroup\$ – Vladimir Cravero Jun 14 '15 at 9:04
  • \$\begingroup\$ Note that your two lumen values are within a factor of 2 of each other, which is fairly close in how humans perceive light. I'd go for the high side of your range since project bulbs are optimized for brightness at the expense of lifetime. \$\endgroup\$ – Olin Lathrop Aug 31 '15 at 10:53

Your calculations are correct, and you should probably figure on about 5000 lumens. Projector bulbs aren't expected to last nearly as long as, say, overhead lights, so they are driven harder for greater efficiency.

However, be warned that getting your optics right may be more of a challenge than you realize. A good condenser works on a small light source, such as the glowing filament, but LED bulbs are a much larger source since they use a phosphor coating on the bulb envelope. This will make it much harder to get good light concentration on the slide, and you may have to use a more powerful LED than you had originally thought.

It's fun idea though. I wish you luck.

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  • \$\begingroup\$ To address the issue of light field uniformity it may very well be suitable to use multiple lower power LEDs placed in an array. \$\endgroup\$ – Michael Karas Jun 14 '15 at 11:59
  • \$\begingroup\$ I know, hence, I need to replace the lens and reflector of the existing bulb assembly. The large area LED is gonna be at least 15mm on each side, and there are reflectors and collimators available for those kind of light sources. I might need a diffuser, though. \$\endgroup\$ – polemon Jun 14 '15 at 14:07
  • \$\begingroup\$ Large area LEDs (rather than free-standing bulbs) need to be mounted on a heat sink, which will be opaque. Mirrors aren't going to help as they do with your existing bulb. \$\endgroup\$ – WhatRoughBeast Jun 14 '15 at 16:40


Apparently, the output of low-voltage halogen is around 24 lumens per Watt. So your halogen lamp is probably putting out around 24*150 = 3600 lumens.

If the current projector is sufficiently bright, I would suggest to keep that illumination level. Halogen lamps can tolerate very high operating temperatures, but LED lamps cannot. If you keep the lumen output the same, then the temperature problem will be more manageable.

Best of luck.

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  • \$\begingroup\$ Well, yes. The conversion table is the one I've linked in my post. Also, that's the one I used... \$\endgroup\$ – polemon Jun 15 '15 at 2:46

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