The current situation outside of large cities in lower-income countries, as described above, is suboptimal for the diagnosis of glaucoma. Doctors have no means of assessing visual fields, and as such may not be able to identify the condition until too advanced for any meaningful intervention (ie, to enable preservation of vision). Vision loss from glaucoma is irreversible. Understanding, through this pilot study, how well the World Sight Foundation Field Machine (WSFFM) works is an essential step towards trialling its use in lower-income countries. The justification for enabling this is to allow clinicians in the developing world to make more informed assessments of their glaucoma patient's status; this should translate in more timely treatments or referral for further investigation before further irreversible vision loss.

The WSFFM will be compared to the Humphrey Visual Field (HVF) analyser. The HVF analyser is the gold standard method of measuring visual fields; in our hospital eye service it is almost exclusively the method used, and as such the field technicians are particularly skilled in their use. The output data of the HVF enables easy quantification of level of field loss; by comparing the output from the WSFFM it would be possible to allow some form of quantification against "the standard" such that an idea of how well it works may be calculated.

This is an exploratory pilot study, to gather information on the use of WSFFM. The investigators do not expect the WSFFM to be equivalent to the HVF and would expect it to be inferior to it; thus, this is not a study of equivalence nor non-inferiority. The trial seeks to instead understand how well the WSFFM works in its purpose of identifying moderate/severe visual field loss in glaucoma patients (which is a different purpose to the HVF analyser, which aims to identify early field loss, as well as monitor for progressive change). The patients the investigators will be investigating will be those attending glaucoma clinic at a single site NHS outpatient hospital eye service. Patients attending glaucoma clinic will have a range of severity of glaucoma (ranging from those in whom a glaucoma diagnosis is still being ascertained, to those with established severe disease) who require assessment by a specialist glaucoma clinician. Patient may not require visual field testing every clinic visit, and not necessarily both eyes (e.g if one eye is blind). The pilot study will be run alongside normal clinical activities. All patients requiring visual fields will be identified in advance of the clinic and approached with a patient information leaflet (detailing the study aims and procedures, and their involvement and rights). Their "trial visit" will be opportunistic as they already will have attended for their appointment; there will be no further follow up after this visit. After their normal initial assessments (as required for all clinical visits), patients will be led to a designated room prior to field testing. Here, an opportunity to answer any queries will be given, before proceeding to consent if the patient wishes to participate. The trial design would most fit a cross-over design. One eye of the participant will be selected at random (unless they were only due for testing of one eye) to have visual fields tested by both devices in a successive time period; the order in which the participants are tested by each device is determined at random. Participants will then undergo visual field testing as per randomisation, for those only clinically requiring one eye testing, they will either undergo WSSFM or HVF testing first, followed by the other (according to randomisation). For those requiring both eyes, the eye to be included will be randomised, in addition to order of device testing for that eye. If the HVF device is randomised first, then one would start with the fellow (non-included) eye to save unnecessary switching between tests.

For example:

• If Right eye included & WSFFM first: Right WSFFM, Right HVF, Left HVF
• If Right eye included & HVF first: Left HVF, Right HVF, Right WSFFM The WSFFM field machine works on a USB connection 5.3 Volt. It can be powered from a phone booster battery or from the USB port of a laptop computer. The machine uses a microprocessor to control stimulus length and brightness.

The patients will firstly be seated in front of the WSFFM which comprises a box with multiple LED lights inside. There is a chin rest and viewing window through which participants are asked to view the lights inside the device. To first ascertain which brightness of light is required, they will be presented with lights of increasing brightness (three levels) until they can see it; this ensures that those with poor vision throughout their whole visual field - e.g. from a cataract, will still be able to be tested for any superadded visual field loss which might be in keeping with glaucoma. There will be a red light at the centre which participants will be then asked to focus on throughout the test to ensure correct fixation. They will then be shown ten different patterns of lights. These lights will light up in a specific sequence and will not always include the same number of lights. Each time a pattern of lights appears, participants will be asked how many lights they can see; if they see fewer lights than presented, they will be asked where the lights are positioned (left, right, up, down). Their responses will be mapped out by the visual field technician to give a representation of visual field in that eye. Participants will then be thanked for their participation in the trial. They will be given a questionnaire (three questions relating to ease of use, and acceptability of the WSFFM) to complete as they wait to conclude their consultation with the glaucoma clinician (or before they leave, if attending virtual clinic, and further consultation on the day not planned).

There would be no further visit nor contact with the participant required or planned.

Inclusion Criteria:

• - Patients attending glaucoma outpatient clinic
• Patients requiring visual field testing, with plan for "SITA-STANDARD" HVF testing
• Patients 18 years or older and of any gender
• Patients capable of giving informed consent.

Exclusion Criteria:

• - Patients not requiring visual field testing on that outpatient clinic attendance.
• Patients requiring visual field testing, with plan for "SITA-FAST" or "SITA-FASTER" HVF testing.
• Patients who do not attend their outpatient clinic appointment,
• Patients not able to perform visual field testing (for example due to their visual acuity, physical difficulty in positioning for test duration, or cognitive difficulties in understanding instructions).
• Patients under the age of 18 years old.