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Phototesting patients

Patient information A-Z

This leaflet is for patients who have been asked to have phototesting.

We are asking you to have phototesting because we think you may have one of the number of skin conditions that are triggered or made worse by the invisible ultraviolet (UV) part of sunlight. Phototesting helps us find out which part of sunlight is causing your skin problem, and how sensitive you are to this. Our dermatologists can then diagnose and treat your skin condition.

The phototesting and receiving your results will take place across two appointments normally on a Tuesday (with the nurse for the testing) and the Wednesday, the next day (with the dermatologist to read the test). It is important that you attend both of these appointments. Your GP (and referring dermatology department) will also be contacted with the results.

What is phototesting?

Phototesting is undertaken with a Monochromator machine. It produces the wavelengths present in the sunlight. Different doses are applied to the back (approximately 5mm in diameter). This helps the dermatologist diagnose which, (if any) parts of sunlight are causing your skin condition.

UVA and UVB will be used during this test, as UV light is the part of sunlight that causes most skin problems. The test is not painful.

While having this test, we will adjust your clothing to expose your back- please wear suitable clothing to allow this. We test small areas on your back, with you lying face down on a couch or sitting up and leaning forward onto a couch. We will mark your back with a skin marker pen to make it easy to read the test the next day by the dermatologists. Please wear suitable clothing to allow for your back to be exposed, but we also suggest you wear dark clothes to avoid the skin marker pen staining pale fabrics.

How long does a phototesting session last?

We normally test with a standard regime that may last between 45minutes to 1 hour. If we suspect a diagnosis of Solar Urticaria this may take up to an hour and a half

What should I bring for my phototesting appointment?

  • List of oral/topical medication that you normally take (including any skin treatments, creams and/or ointments).
  • Any pictures you have of your skin condition (if not already seen by the dermatologist).
  • You are welcome to bring something to read or listen to while having this test.
  • Please stop applying any steroid cream to your back three days before your Phototesting appointment. Moisturisers should be stopped the day before.
  • You will need to keep your back dry after the phototesting to avoid the marker pen being washed off until the dermatologist has looked at the reactions the following day.
  • Children (under 16 years of age) must be accompanied by a parent/guardian.

What are the benefits of Phototesting?

  • Phototesting helps us to diagnose photosensitivity and to discuss possible treatment.

What are the risks of Phototesting?

  • If you are sensitive to sunlight, you will usually develop the same skin reaction that you have to sunlight but only on the small sites we use for phototesting.
  • The doses of ultraviolet and visible light we use for testing are roughly the same as those experienced within 20 minutes of exposure to UK summer sunshine. In healthy skin, it is normal for the longest exposure dose of photo testing to turn the skin slightly pink or produce a tan. The reactions normally fade over a few weeks.
  • If you experience a strong response to the tests, the consultant will prescribe a steroid cream, which you should apply to the skin as directed, to reduce the reaction.

Where is the test performed?

The test is performed in one of the treatment rooms in clinic 7 outpatient department level 3.


If you need to cancel the appointment please ring clinic 7 on 01223 216234; or if you have any concerns or questions about your phototesting appointment please call the secretary (number at the top of your clinic appointment letter) of the Doctor whom referred you for photo testing and leave a message and you will be contacted as soon as possible.

We are smoke-free

Smoking is not allowed anywhere on the hospital campus. For advice and support in quitting, contact your GP or the free NHS stop smoking helpline on 0800 169 0 169.

Other formats

Help accessing this information in other formats is available. To find out more about the services we provide, please visit our patient information help page (see link below) or telephone 01223 256998.

Contact us

Cambridge University Hospitals
NHS Foundation Trust
Hills Road, Cambridge

Telephone +44 (0)1223 245151