LEDs: Quality Over Quantity of Light
Pulse
Octopart Staff
Oct 10, 2019

II 18 wide

If you’ve ever designed LED lighting, you’ve probably come across some very expensive LEDs advertising high Colour Rendering Index (CRI), and perhaps thought the cheaper LEDs would be better, allowing you to pack in more light output for the same amount of money. If you, however, are just about to design LED lighting and aren’t really familiar with colour rendering indexes, then this might be a great time to learn about it. How much does the colour rendering index of a LED really matter after all?

Depending on your application, the colour rendering index could make the difference between a great product and a pretty terrible one. If your application involves food, retail displays, photography, or videography, then colour rendering index is absolutely vital, more so than absolute brightness.

As an amateur photographer, I tend to design personal projects using 97+ CRI white LEDs. Being personal projects, the cost difference between a high CRI LED and a regular white LED isn’t great, and it gives me satisfaction knowing the light is going to make colours look beautiful. Not everyone has the same appreciation for vivid, vibrant colours achieved through expensive high CRI LEDs that I do, and I hope to change this through some tests in this article.

I’ve seen some videos on YouTube about making big, bright DIY panel lights using cheap LED strips. The LED strips they advertise are indeed very cheap, however, the quality of light is too. When I was working in a research lab several years ago, I specified a strip of 95+ CRI LEDs for a LightBoard project that was to be used internally. Some of my colleagues baulked at the cost of the LED strip, which, at the time, had cost almost 20 times more than the cheapest LED strip of the same length sold on online marketplaces. My arguments for quality of light won out, and the project ended up looking spectacular with very bright vibrant writing on the light board, which far surpassed anything the cheap strips could produce.

At the time, I didn’t have the chance to demonstrate with a side by side comparison of the expensive light strip versus the cheap one, so instead, I set out to share my love of high quality LEDs here.

I headed to Amazon and purchased the cheapest 5 metre LED strip I could find. My purchase was for ‘12V LED Ribbon Waterproof Flexible LED Tape 5050 SMD 300 LEDs’.

Then I started scouring UK lighting supply websites for ‘High CRI LED strips’. The only thing I was interested in was the colour rendering index, not brightness, voltage or current. I was hoping to find some daylight/neutral white LEDs so I could re-use them in another project. At about 15 times the price of the Amazon LEDs, I could have purchased some 96 CRI LED strips, but the supplier was out of stock. I went with the cheaper 90 CRI LED strip that worked out to only around 5 times the price of the Amazon LEDs, as it only came in a 10 metre pack. I was hoping for 95+, but not having found any, I hoped that 90 CRI would be enough to make my point.

What is Colour Rendering Index

At this point, you might be wondering what colour rendering index is, and why I’m so enamoured of it. Colour rendering index is a measure of the quality of artificial white light, with sunlight at 100% being the gold standard it is compared to. Sunlight is what our eyes are adapted to, and it provides a very even spectrum of light for us to see with. CRI is calculated from comparing a range of CIE standard colour samples with a reference light source of the same colour temperature, and then looking at the percentage of difference. My 90 CRI LED strip is missing about 10% of the visible light spectrum compared to the sun, which is why I would have much preferred a 95+ one.

Colour rendering index is independent of colour temperature. A cheap cool white LED is typically going to have a lower CRI than a cheap daylight colour temperature, as the daylight LED naturally needs more red light. A good quality cool white LED can still have 95+ CRI, however, as it just requires more specialised phosphorescent material from the manufacturer.

Low cost LEDs tend to have low quality light output, as it requires a specialised blend of different phosphorescent materials to create all of the wavelengths of light required to see all colours naturally. You can create a white LED yourself using a single RGB LED by simply turning on all the dies at once. This creates a very low quality light as you would have experienced yourself, as the light only has 3 points on the spectrum - red, green and blue.

led1Zhang Junchao, Snapshot multi-wavelength interference microscope. Optics Express. 26. 18279. 10.1364/OE.26.018279.

To our eyes, which see in red, green, and blue, this looks like white light. However, reflections off something as simple as a red apple don’t look right. The apple looks dull, lifeless, or perhaps a bit sickly because there’s only one small part of the red spectrum reflecting back from it.

If we compare our very poor light quality RGB example to sunlight, you can immediately see why it’s a poor substitute.

led2Source: Ultraviolet damage to the eye revisited: Eye-sun protection factor (E-SPF®), a new ultraviolet protection label for eyewear. Clinical ophthalmology (Auckland, N.Z.). 8. 87-104. 10.2147/OPTH.S46189.

To test the colour rendering index, a lab will shine the light source under test on 15 test colour samples, the reflection of which is then measured and compared to the reference light source (the sun). This gives an R value for each colour swatch, which is how faithfully a light source under test can render that colour. CRI is the Ra, or the average reflection of all the swatches, so you may see CRI listed as Ra by some manufacturers, or using Ra as the ‘units’.

Generally, 95 CRI is considered ‘high’. However, some manufacturers might stretch this down to 90, as the manufacturer of my relatively cheap ‘high CRI’ LED strip has done. Very cheap LED strips are around 30-50 CRI, and you can generally consider a manufacturer who will not advertise their white LEDs’ CRI to be fairly poor.

If you use a low CRI light, objects won’t look as a viewer expects. If you’re building lighting for a restaurant or the produce section of a supermarket, customers might feel that the food looks off, or perhaps not very high quality, because it doesn’t look like they expect.

If you’re designing lighting for photography or videography, a low CRI light source will result in dull colours, and skin tones that make people look ill. There’s not a lot you can do in post production to fix this either, as those wavelengths of light simply were not present for the camera to capture.

Test Setup

Now that you know what CRI is, let's get back to testing. I have taken the anti-static mat off my partially set up lab desk to form the test area. It’s night, and there is no source of light except the LEDs and the relatively dark lab power supply screen. I have taken approximately 900mm of LED strip of each 5 metre reel for the test.

Cheap LED strips vs 90+ CRI LED stripTesting the quality of light using high quality and cheap LED strips lighting.

The test setup is simple: my Rigol DP832 power supply is powering each LED strip, which is attached to a small tripod at one end and taped to my magnifying lamp at the other end. Both LED strips are taped together, so they follow the same path, which will result in equal lighting between the strips. Each test subject will be photographed with each LED strip lighting it, resulting in two photos of each test subject, one lit by the cheap LED strip, and one lit by the 90+ CRI LED strip.

My test subjects are:

  • Cheap coloured pencils from Amazon, with the widest colour range possible.
  • A Datacolour SpyderChecker 24 colour calibration chart.
  • Cheap set of art pastels.

The colour calibration chart is a high quality industry standard chart that is used for colour correcting photo and video in post production. It’s not the same colours used for CRI testing, but it does offer high quality reference colours for us to look at.

For the camera, I’m using a Nikon D5600 taking raw 14 bit images, with a Sigma 18-35mm F/1.8 lens at 22mm of zoom, f/6.3. Whilst the camera only sees a specific band of red, green, and blue, after looking at the test photos, I would say the differences in just switching between the two LED strips are even more apparent to the naked eye. The photos were edited to ensure the exposures were correct, and to ensure the white balance was set to the third grey level of the colour test chart in DXO Optics Pro 11. As you will see in the test photos, the cheap LED strip was starting to struggle beyond the third grey level.

Since we’re not interested in the quantity of light in this test, the fact that the light is not evenly distributed across the test subjects is irrelevant. We are only interested in what colours the camera can see. In the test images, I have attempted to match the brightness of mid blue colours between the two images as these are the colours most equally rendered by both LED strips.

Test Images

White balance setting of Cheap LED stripMy white balance settings for the cheap LED strip came out as 8675 Kelvin, with -66 tint.

The expensive LED strip was 4295 Kelvin (advertised as 4000K), and -43 tint.

Test Chart

led6Test chart comparing cheap LED strip with expensive 90+ CRI LED strips.

The top-left corner of each box is the cheap LED strip, and the bottom-right colour is the expensive LED strip that claims to be 90+ CRI. The top two rightmost boxes are skin tones, the bottom row should be perfect greys which reflect the entire visible spectrum equally.

You can see in the test chart that the red box is very dark when subjected to the cheap LED strip, as there isn’t much light to reflect back. The pure blues look very similar for both, confirming a good white balance and fairly close exposure match between the two images.

Pastels

Testing the quality of light using pastels.Notice that the top right colours look very dull and lifeless.

For the pastels, I used the same white balance settings that were determined with the test chart. The split for these images is the same, with the cheap LED strip in the top right and the expensive high CRI LED strip in the bottom right.

Again, we see the bright yellows look very dull and lifeless on the cheaper LED strip. The pinks under the low cost LED strip look almost purple. Imagine how bacon, ham, or a fresh chicken breast would appear!

Pencils

Testing lighting quality using coloured pencils.The difference for most colours is night and day.

For this image, I went with alternating rows of high CRI and low CRI LED lighting. I didn’t quite get the pencils lined up perfectly due to the camera being slightly tilted on the cheap LED strip photo, however they should line up well enough for a comparison.

I love how this image shows most blues to look very similar, yet the purples, which need a bit of red to look purple, and the pinks, look really off. Yellows, again, look very dull and lifeless, and the reds very dark.

Thoughts on the Test Images

I haven’t ever tested LED strips like this as a side-by-side comparison before, and the difference when testing was very jarring. The cheap LED strip looks very wrong to your eyes, and looks significantly darker even when consuming the same current as the more expensive strip. Despite the expensive strip only being 90 CRI, it still looks and feels very welcoming and pleasant to the eyes. It feels like stepping out of a fluorescent-lit office on a spring day, when the sunlight isn’t too bright but just warm enough to be pleasant. The low cost LED strip feels like going back inside to work on a report you’ve been procrastinating over for weeks.

The camera captures a lot of the difference in light quality, but it really doesn’t capture it all. Looking at the images on my computer, I can see the colours from the cheap LED strip don’t look quite right, and the images look dull and lifeless. Our eyes have a much better perception of colour than the camera does, and the more complete light spectrum looks significantly brighter and more pleasant to the eyes.

These emotions and feelings would influence a retail customer’s feelings of a product that is on display too. If you were displaying a beautiful object in a store under the light of the cheap LED, it wouldn’t be as attractive or enticing as if it were lit by the higher quality LED strip. As an engineering firm, your potential customers are the retail stores; your job is to make their products look as good as possible to entice visitors to make purchases at the stores. If you use a large amount of poor quality lighting in your device, it may be very bright but not sell the product.

Likewise, if you were designing a bespoke lighting solution for a bar or restaurant, you want to ensure that every bit of light reflecting off what the patron is consuming makes it look appetizing and tasty.

Good lighting matters for more than just retail, produce, and restaurants. If you are designing lighting for colour machine vision, ensuring the lighting is good quality will give the computer brighter, more vibrant colours to work with, which will in turn improve the quality of the machine vision algorithms.

Conclusion

Next time you’re shopping for white LEDs, consider selecting based on the colour rendering index of the choices to ensure what you are lighting looks vibrant and beautiful to the viewer. Octopart has a filter for CRI, and if you click here, I have pre-filtered all the LEDs that Octopart knows about to just show 90 to 100 CRI LEDs. Now, you have a great list of LEDs, which you can begin sorting by price and filtering for luminous flux or luminous intensity.

When choosing parts for a project, there are many times at which signal quality is significantly more important than signal quantity. We look at signal-to-noise ratio in many devices we use, and optimise for the highest performance parts we can fit into the budget. When it comes to lighting, the sensor we’re optimising for is the human (or robot!) eye. CRI is just one of many factors when it comes to selecting a great LED for your project, but I feel it may be one of the most important, as it can have so much influence on the viewer's feelings and emotions when viewing the lit subject.

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