Light test

When performing a light test, ultraviolet light is used to see if sunshine is the cause of skin rashes.

What is a light test?

Light tests are used to check if sunshine may be the cause of skin rashes.

For a light test, we use ultraviolet light, which is a component of natural sunlight.

Ultraviolet light consists of UVB, which is theshort wavelength part and UVA, which is the long wavelength part of ultraviolet light.

How are light tests conducted?

We use the lower part of your back, i.e. just above the buttocks, for the testing. 

Four small areas are exposed to UVA and five small areas to UVB.

We use a marker pen to mark the boundaries of the exposed areas.

Already after 20 minutes, we take the first control readings of the results.The day after the light test, we check your skin to see if the exposed areas have become red or swollen. This we do again 3-7 days after the test. After the final control reading, an explanatory talk will be scheduled with a physician.


Avoid taking antihistamines four days prior to the testing. Also, do not apply cream or ointment to the small of your back.You may treat the upper part of your body as you would normally do. 

If you have been undergoing tar therapy, at least one week must pass before the light testing can be performed as the tar will make your skin more sensitive to light.

If you 14 days before the test has been treated with Prednisone, as tablets or injections, please let the clinic now before the test.

Please tell us about any drugs you may be taking in that many drugs can alter the photosensitivity of your skin, which could affect the light test results.

The testing cannot be performed if you have a tan due to sunbathing. Hence, avoid exposing you skin to the sun four weeks prior to the light test. This includes natural sunlight and tanning beds.Do not apply cream or ointment to the lower part of your back until after the testing.

What are the side effects?

The only known side effect is discoloration of the skin in the exposed areas. This discoloration may remain for a prolonged period of time but will fade eventually.

Meeting points

Upon arrival, please report to the dermatology clinic (Hudsygdomme, klinik 1), entrance 15, ground floor.

The actual testing is performed at the allergy testing clinic (Kontaktallergi, klinik1), entrance 15, 2nd floor.