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Photosensitivity is the amount to which an object reacts upon receiving photons of light.

People that are photosensitive experience discomfort or get easily sunburned when exposed to UV light, which may come from sunlight or other sources including sunbeds. This is often caused by an allergy or a medication.

Certain electronic devices, such as photodiodes and charge-coupled devices, are designed to be sensitive to light. They are constructed to take advantage of the photoelectric effect, the emission of electrons from matter upon the absorption of electromagnetic radiation. When light (one form of electromagnetic radiation) impinges on the active surface of such a device, electrical current flowing through or electrical charge stored in the device will increase or decrease in proportion to the intensity and wavelength of the light. This trait allows the device to perform regulating and sensing functions of many kinds. For example, a photoresistor circuit may sense ambient light to turn on a street lamp at dusk. Digital cameras use charge-coupled devices whose extreme sensitivity to light allows them to convert incoming photons into varying electrical charges with great accuracy. The varying charges are then encoded in a binary file which can be stored and later viewed on a computer screen or other medium.

Chemicals that are photosensitive may undergo chemical reactions when exposed to light. These chemicals, such as hydrogen peroxide and many prescription drugs, are stored in tinted or opaque containers until they are needed to prevent photodegradation. Devices that are photosensitive include the human retina and photographic film. Film is photosensitive because its photosensitive material undergoes a chemical reaction when struck by light.

Typical substances that are photosensitive are alkali salts and silver halides.

• Heliotropism
• Photophobia
• Photosensitivity in animals

• FDA article - Chemical Photosensitivity: Another Reason to Be Careful in the Sun

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