The purpose of this clinical trial is to evaluate the effect of different intensity of repeated low-level red-light (RLRL) therapy on the choroidal and retinal blood flow among adults.

Repeated low-level red-light (RLRL) therapy is an emerging innovative and non-invasive treatment for a variety of eye diseases. Notably, RLRL was found to be effective in thickening choroidal thickness in a 1-year randomized controlled trial, indicating its potential in modulating blood flow in the fundus. However, it remains unclear how long it takes for the RLRL to make a difference in fundus blood flow and whether there is a dose-response.

The purpose of this study is to evaluate the different wavelengths of of repeated low level light therapy on the choroidal and retinal blood flow among adults. This study will be conducted with a randomized cross-over design with a total follow-up of 3 months. The repeated low level light therapy will be carried out in the study site under supervision according to a standard protocol. Detailed ophthalmic examinations, including visual acuity, intraocular pressure, optical coherence tomography/angiography, optical coherence tomography angiography, cycloplegic spherical equivalent refraction, slit lamp, and biological parameters will be evaluated at enrollment and during follow-ups.

Inclusion Criteria:

1. Age: 18-40 years at enrolment.
2. Healthy adults with best corrected visual acuity equal to or better than 1.0 in both eyes.
3. Provision of consent and able to participate in all required activities of the study.

Exclusion Criteria:

1. Secondary myopia, such as a history of retinopathy of prematurity or neonatal problems, or syndromic myopia with a known genetic disease or connective tissue disorders, such as Stickler or Marfan syndrome; or high myopia < -8 diopters.
2. Strabismus and vision abnormalities (astigmatism > 3.5 D) in either eye.
3. Refractive media opacity: corneal opacities, cataracts, or implanted intraocular lens, etc.
4. Ocular abnormalities that affect retinal function: macular degeneration, diabetic retinopathy, retinal detachment, glaucoma, or ocular hypertension, endophthalmitis, uveitis, optic neuropathy, etc.
5. Previous history of refractive surgery, intraocular surgery, laser therapy, and intravitreal injection, etc.
6. Systemic abnormalities: diabetes, hypertension, etc.
7. Drugs therapies with toxicity effects on the retina: hydroxychloroquine, etc.
8. Prior treatment of myopia control in the past three months including drugs, orthokeratology, progressive addition lenses, bifocal lenses, etc.
9. Other contraindications, including but not limited to ocular or other systemic abnormalities, that the physician may consider inappropriate for enrolment.