This study will assess the effect of 2 different colours of light on colour vision in adults with retinitis pigments (RP). Participants will be 18 years or above with a genetically confirmed molecular diagnosis of RP. After informed consent, participants will have their letter chart vision and colour vision measured in their study eye. They will then be they will be randomly allocated to one of 2 groups. The study team will not know which group they have been assigned to. Group 1 will be given a coloured-light hand-held torch and be asked to apply it to their study eye for 3 minutes at home each morning between 8 - 10 am for 28 days. Group 2 will be given a different colour hand-held torch and be asked to do the same. All participants will have their letter chart vision and colour vision measured at the end of study on day 28 (+ 7 days).

This is a pilot, randomised, masked, single-site trial designed to assess the effect of coloured light exposure on cone contrast thresholds (CCT) in adults with RP.

70 adults with typical RP in which the underlying genetic cause is known will be recruited from Moorfields Eye Hospital genetics service.

Following informed consent, participants will be invited for a baseline study visit.

At a baseline visit, prior to any light exposure (day 0), participants will undergo Logarithm of the Minimum Angle of Resolution (LogMAR) visual acuity (VA) and CCT measurements performed by an experienced technician according to dedicated standard operating procedures. The order of testing will be as follows:

• Study eye: LogMAR VA
• Study eye: CCT (Tritan and Protan)

After baseline assessments have been completed, participants will be randomised to the intervention (n=35) or control group (n=35) by a study nurse. All other study team members will be masked to group allocation.

The intervention group will be provided with a coloured torch and instructed to apply it to their study eye for 3 minutes at home each morning between 8 - 10 am for 28 days. The control group will be provided with a different coloured torch and will be instructed to follow the same protocol. The specific colours have not be disclosed here to protect masking of the study. Participants will be asked to complete a paper diary each day to record whether light was applied, the time and duration of the application and the eye to which light was applied.

The end of trial visit will be scheduled on day 28 (+7 days). Participants will undergo LogMAR VA and CCT measurements by an experienced technician according to standard operating procedures (SOP). The order of testing will be as follows:

• Study eye: LogMAR VA
• Study eye: CCT (Tritan and Protan)

Inclusion Criteria:

• Over 18 years old
• Typical RP where the underlying genetic cause is known. This may include PRPF31, PRPF8, SNRNP200, RHO, RP1 [autosomal dominant] and USH2A, EYS [autosomal recessive]
• VA of 0.30 LogMAR (6/12) or better in the study eye
• Preserved foveal structure defined by intact outer retinal (ellipsoid zone) at the fovea on Optical Coherence Tomography (OCT) and a hyperfluorescent ring within the temporal vascular arcades on Fundus Autofluorescence (FAF) in the study eye
• Willing and able to provide written informed consent

Exclusion Criteria:

• Co-existing ocular pathology will be excluded; however, participants with macular oedema, a common pre-symptomatic manifestation of foveal involvement, will not be excluded
• Concurrent and/or recent involvement in other research that is likely to interfere with the intervention within 6 months of study enrolment
• Unwilling or able to provide written informed consent.

Data will not be shared with other researchers

Technicians responsible for study measurements, Chief Investigator (CI), Principal Investigator (PI) and collaborators will be masked to the treatment allocation. It is not possible for participants to be fully masked as they will know which wavelength (colour) of light they have been provided, but they will not be told which wavelength of light is the active intervention. Participants will be advised not to discuss the colour of light used with the masked technicians and study team members.