How can you tell if lights emit harmful blue wavelengths at night?

Diffraction glasses are an inexpensive & simple way to determine whether circadian blue is emitted by a yellowish or white light.

Electric lights which emit too much sky-blue light in the evening, and provide too low a dose of circadian blue light during the day, are the key disruptors of our circadian clocks, and the cause of significant chronic ill-health.

But how can you tell whether an electric light is blue-free or blue-rich? Most lights look yellowish-white, so you cannot easily tell at a glance whether a light bulb, or a light fixture, is healthy or not.

How to find out whether a light emits blue

The gold standard for measuring the circadian blue content of lights is a handheld spectrophotometer, but that can set you back $1,000 or more, so it is only practical for lighting professionals.

But there is a convenient and inexpensive solution - diffraction glasses - that anyone can use and that gives you a reasonable approximation. They are often made with paper frames and are available on the web for a few dollars. The example shown below is used by Korrus¹ to educate customers about circadian lights, and let them witness the difference between blue-rich day lights and zero-blue evening lights.

The paper frame diffraction glasses used by Korrus to show their customers the difference between their max blue and zero blue circadian lights

When you look through these diffraction glasses you see, off to the side, the rainbow spectrum emitted by the light. The sample conventional LED light bulb, shown below, can be seen to be rich in sky-blue wavelengths. This would not be an appropriate light for evening use.

Representation of what you see when you look through diffraction glasses at a light bulb that is rich in sky-blue wavelengths. Off to the side of the light source appears a spectrum of the light wavelengths that comprises the white light. The sky-blue band disappears with lights that have zero circadian-blue content.

Of course, a spectrophotometer will provide precise measurements of all the visible wavelengths emitted by the light bulb. Most spectrophotometers provide a visual plot on the screen as shown below, so you can immediately see the relative power of every color wavelength.

A handheld spectrophotometer shows that the light emits a large spike of blue light and would be harmful if used after sunset

You can also export the spectral power data from the spectrophotometer into an Excel spreadsheet for every visible wavelength from 380 nm (violet) to 780 nm (red). Then the percentage of circadian blue in the total visible light can be simply calculated as (sum 440-495) / (sum 380-780).

Manufacturers fail to warn the consumer

Unfortunately, manufacturer’s labels usually aren’t helpful. They should say whether a light has less than 2% blue content and therefore is suitable for nocturnal use, or has greater than 20% blue and is therefore optimal for daytime use. But very few manufacturers report blue content. Manufacturers should also provide the spectral power distribution (“SPD”) of the light in their spec sheets (i.e. the relative amount of power at each wavelength in the visible light spectrum). But many fail to do this.

The reason is simple. Most lights sold today emit unwavering levels of circadian blue that are too low during the day, and much too high for evening or nocturnal use. And the manufacturers don’t want to admit that.

Scientific Consensus - Most LED Lights should have a Warning Label

Recently 250 leading circadian scientists, who had published the most peer-reviewed papers on circadian clocks and light, reached a consensus².

LED lights with high 460 - 495 nm blue content should carry the warning label “may be harmful if used at night.”

This was based on their expert evaluation of an extensive scientific research literature of over 10,000 scientific papers on light and circadian clocks, and the impacts on human health and disease.

How to get a campaign for healthy lighting started

We are launching a movement to convert the world to healthy lighting starting with our own homes and workplaces. Already health-conscious people around the world are joining the Campaign for Healthy Lighting.

It all starts with education:

1. Buy copies of THE LIGHT DOCTOR and share them with your friends and colleagues.

Give the book to people who need to hear and know about healthy lighting. It is the essential guide to healthy lighting.

Buy single copies on Amazon for $19.98 ( 33% discount from the list price)

SPECIAL OFFER: Buy 5 or more copies at $9.99 each to help get your own healthy lighting campaign started

2. Subscribe (free or paid) to this THE LIGHT DOCTOR Substack newsletter

This newsletter will keep you updated with the latest research findings and conclusions on light and health, and will evaluate the new circadian health products that are being introduced.

3. Join the Campaign for Healthy Lighting

Become part of the movement for healthy lighting You can sign up at https://circadianlight.org/campaign/

1. Disclosure: Korrus acquired the circadian lighting patents which were based on the research our team conducted by the Circadian Light Research Center. These patents underlie the Korrus Soraa zero blue light bulbs and the Lumium Circadian Blue light fixtures. Circadian Management Inc. which operates the Circadian Light Research Center holds a small minority equity stake in Korrus, and has a sales representation agreement with Korrus for some circadian lighting products. However, our mission at the Circadian Light Research Center is to educate the world about all forms of evidence-based circadian lighting whether or not they are manufactured or sold by Korrus.

2. Moore-Ede M et al (2023) Lights should support circadian rhythms: evidence-based scientific consensus. Front. Photon 4: https://doi.org/10.3389/fphot.2023.1272934