Oh, Holey Light

We consider ourselves well-versed when it comes to the technical literature plastered on hardware store parts. Acronyms don’t frighten us, and our Google-fu is strong enough to overcome most mysteries. One bit of dark magic we didn’t understand was the gobbledygook on LED lamps. Wattage is easy and color temperature made sense because it corresponds with warm and cool colors, but Color Rendering Index (CRI) sounds like deep magic. Of course, some folks understand these terms so thoroughly that they can teach the rest of us, like [Jon] and [Kevin], who are building a light controller that corrects inadequacies in cheap lamps by installing several lamps into one unit.

We learned a lot by reading their logs, which are like the Cliff Notes from a lighting engineer’s textbook, but we’ll leave it as an exercise for the students to read through. Their project uses precise light sensors to measure the “flavor” of light coming off cheap lamps so you can mix up a pleasing ratio. In some ways, they are copying the effects of incandescent bulbs, which emit light relatively evenly across the visible light spectrum, right into the infrared. Unfortunately, cheap LEDs have holes in their spectrum coverage, and a Warm White unit has different gaps compared with Daylight, but combining them just right gives a rich output, without breaking the bank.

18 thoughts on “Oh, Holey Light

  1. That three color sensor isn’t giving any useful information about the spectral coverage of the bulbs. It is no better than your eye. An actual spectrometer is needed, along with a calibrated white sphere to ensure that the light isn’t being affected by the surroundings.

    1. Well, I don’t know about your eyes, but mine don’t output 16 bit numbers which correspond to measurable amounts of red, blue, green and IR light. I just get varying levels of “looks good,” “too dark,” and “I think it looks (color)ish.” Oh yeah, and I can also manage, “No I still can’t see IR.”

      1. As an electrician, I’ve had the opportunity to judge many different types of lighting. I still prefer halogen lights when it comes to illuminating artwork, nice stone countertops, or anywhere color depth and accuracy is appreciated. In that sense, fluorescents have always sucked, LEDs used to suck hard, but are getting MUCH better lately. Ceramic metal halide lamps are also very good for CRI.

        I’m looking forward to the day when inexpensive LEDs have a great CRI. Right now they’re “close”. Put cheap LEDs in your clothes closet and compare them to a halogen or CMH lamp and it may change your idea of what “looks good”.

      2. Yes, but your eyes are calibrated differently to mine, and we also notice how light reacts with different pigments and dyes it shines on.
        So a spectrometer is needed to check the lights’ output.

        Paint manufacturers do a lot of work to check how things look in different lighting.

        If you get a car panel replaced and they don’t use the original paint, you can find oddities like it matches in daylight but not at night.

  2. I always translated CRI to how tasty my pizza looks under the light. I have tested a few different LEDs with different CRI and noticed the higher the CRI, the tastier the pizza looks.
    Simple quantification, easily understandable for Average Joe, just not scientifically.

  3. Grumble. Seems there’s a range of just how red/green colour blind/deficient you can be. I’m at the wrong end of that range.

    Still, lights with a good colour spectrum do make rooms feel more natural.

    For a shed workshop, a combination of inexpensive warm and cool white LED strips were used to great effect. Neither set on their own looked nice. People with more functional eyes agreed too.

    1. I put up 18 6000K cheap LED 4ft lights in my workshop earlier this year, but have noticed the CRI isn’t great (no surprise really.)

      I’d been toying with the idea of buying a pack of the same lights but in 4000K to mix in for a while now, and you just convinced me. Now I just have to decide if I just want to have 26 light fixtures in here, or if I want 10 of the 5K and 8 of the 4K. Guess I’ll play around when they get here.

      1. *Just looked it up, and it seems I was off on the temperatures of what I have.

        18x 6500k is what I have, and I just ordered 8x 4000k. Hopefully that will be some improvement.

    2. I remember suggesting to customers that we put 1/2 warm white and half cool white fluorescents in their office light fixtures. I’m guessing that would equate to around 3000k, which is interesting as I think about that because I usually recommend 3000k for residential task lighting for things like kitchen under cabinet fixtures and laundry lights…
      All that said, I still use incandescents in my living room cans… nothing beats the low glow of an incandescent in the evening. Philips warm glow is the best led bulb series on the market imo..
      I did buy some drafting fluorescent tubes once for a customer… 6500k and a 96 (or maybe even 98)CRI… mam did those ever pop!

  4. Swapped the four old 4 foot T12’s in the kitchen for some T8 warm white LED universal tubes from Walmart. Great Value house brand, 2 pack. A bit softer than the old fluorescent tubes but much brighter. $8.88 for a 2 pack in warm or daylight. $42.88 for a 10 pack in daylight. They don’t have warm in a 10 pack for some reason.

    They work with magnetic or electronic ballasts. At 17 watts per tube if you have a place with a lot of 4 foot tubes the power savings would quickly pay for these VS the 40 watt T12 or 32 watt T8 fluorescents.

    1700 watts per 100 tubes for LED VS 3200 watts per 100 tubes for T8 VS 4000 watts per 100 tubes for T12. Some 4 foot T12’s go as high as 48 watts.

    I see 8 foot T12 ranging from 75 to 110 watts with the green ended “high efficiency” T8 at 59 watts. I’m finding 8 foot T8 LED as low as 33 watts. The savings in a business or industrial building with 1,000 110 watt 8 foot T12 tubes switching to 33 watt LED tubes would be 77,000 watts. Even switching from the “super efficient” 59 watt 8 footers would cut 26,000 watts of power use. Even at the cheap 6 cent a KWH cost for electricity where I live 26,000 watts saved on lights that are on 24/7 would see a savings of $13,675 per year switching from 1,000 of the most efficient 8 foot fluorescent tubes to 33 watt LED tubes.

    Set typical appliance to heater and use to 24 hours per day. https://www.rapidtables.com/calc/electric/electricity-calculator.html

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