Coloring of Optical Lenses and Wavelength-Specific Filters

Key points at a glance

Key Points at a Glance

• Tinting of optical lenses and wavelength-specific filters are optical techniques to reduce photophobia (light sensitivity).
• FL-41 lenses were developed in the late 1980s and selectively block blue-green light around 480 nm.
• Melanopsin (λmax≈482 nm) in intrinsically photosensitive retinal ganglion cells (ipRGCs) is involved in the development of photophobia.
• Main indications include migraine, benign essential blepharospasm (BEB), post-concussion syndrome (PCS), cone disorders, and eye pain.
• In Japan, light-filtering glasses are used clinically for blepharospasm and cone dystrophy.
• Most studies report subjective symptom improvement, but sample sizes are small and the level of evidence is limited.
• Commercially available FL-41 lenses vary in quality, so verifying the supplier is important.

1. Optical Lens Tinting and Wavelength-Specific Filters

Optical lens tinting and wavelength-specific filters are optical technologies aimed at reducing severe discomfort caused by photophobia (light sensitivity).

FL-41 lenses are considered the first successful example in this field. Developed in the late 1980s, they were designed to reduce discomfort from fluorescent lighting and improve workplace productivity ¹⁾. The name derives from their development history, and they are a lens tinting technology that selectively blocks blue-green light around 480 nm ²⁾.

While simple tinted lenses (sunglasses) reduce light intensity across all wavelengths, both FL-41 lenses and optical notch filters differ by selectively blocking specific wavelengths.

More recently, optical notch filters have been developed. By applying a thin film coating to the lens surface, it is possible to block specific wavelengths more precisely than FL-41 lenses³⁾.

Clinical use in Japan: In the treatment of blepharospasm, light-filtering glasses are tried for patients who experience light-induced spasms or photophobia. Also, for patients with cone dystrophy who have severe photophobia, wearing light-filtering glasses is considered effective in reducing symptoms.

Q What is an FL-41 lens? How is it different from ordinary sunglasses?

A

FL-41 lenses are tinted lenses that selectively block blue-green light around 480 nm, fundamentally different from simple dark lenses. While ordinary sunglasses reduce light intensity evenly across all wavelengths, FL-41 lenses specifically target and block the particular wavelengths that cause photophobia. For detailed mechanisms, see the “Pathophysiology / Detailed Mechanisms” section.

2. Main Symptoms and Clinical Findings

Subjective Symptoms (Symptoms Targeted by Tinted Lenses)

• Photophobia: Significant discomfort caused by light. This is the main indication for this technology.
• Photophobia and ocular dryness: Patients with blepharospasm often experience photophobia and ocular dryness.
• Worsening of spasms: Blepharospasm spasms worsen in bright light, with fatigue, and during reading, and improve in dark environments.

Clinical Findings (Target Diseases)

The main diseases for which wavelength-specific filters are indicated are as follows:

Migraine

Chief complaint: Photophobia is a major accompanying symptom

Recommended filter: FL-41 lens (blocks around 480 nm)

Evidence: Efficacy has been demonstrated

Benign essential blepharospasm

Chief complaint: Light-induced involuntary eyelid closure

Recommended filter: Blue-green light blocking lens / tinted glasses

Evidence: Efficacy has been demonstrated

Post-Concussion Syndrome

Chief complaint: Various visual symptoms including photophobia

Recommended filter: FL-41 lens

Evidence: Improved subjective comfort reported (small trial)

Cone Disorders and Others

Cone photoreceptor dysfunction: Photophobia is prominent. Red contact lenses show the most convincing improvement.

Eye pain: Corresponds to exacerbation of pain due to light stimulation.

Visual snow syndrome (VSS): FL-41 glasses are considered effective for photosensitivity.

In achromatopsia, photophobia is also a characteristic symptom, and light protection measures are important.

3. Causes and Risk Factors

Mechanisms Causing Photophobia

Intrinsically photosensitive retinal ganglion cells (ipRGCs) play a central role in the development of photophobia.

• Melanopsin photosensitivity: The melanopsin photopigment in ipRGCs has peak sensitivity to wavelengths around 481 nm (blue-green light) ²⁾. Light of this wavelength causes isomerization of melanopsin.
• Bistability: Melanopsin is bistable, isomerizing at two wavelengths: 481 nm and 587 nm.
• Input to nociceptive centers: Stimulation of ipRGCs is associated with nociceptive centers in the thalamus, and the 481 nm component of ambient light may activate pain centers in the thalamus.
• Independent light signal transmission: ipRGCs can transmit light signals even in the absence of rods and cones, and are involved in circadian rhythm, pupillary light reflex, and thalamic nociception.

Based on these mechanisms, blocking wavelengths around 481 nm reduces ipRGC light signal transmission and suppresses input to thalamic nociceptive centers, which is the theoretical basis for wavelength-specific filters.

Prevention and Daily Care

If you experience photophobia, it is important to see an ophthalmologist for a thorough examination of the underlying cause. While FL-41 lenses and tinted glasses are commercially available, it is recommended to consult a specialist before purchasing. Using appropriate filter lenses both indoors and outdoors can help reduce symptoms.

Q Why do only specific wavelengths of light cause discomfort?

A

Because melanopsin in ipRGCs has peak sensitivity to blue-green light around 481 nm, this wavelength band is thought to selectively activate the thalamic nociceptive center. Therefore, selectively blocking light around 480 nm is considered more effective in reducing photophobia than uniformly reducing overall light intensity.

4. Diagnosis and Examination Methods

When determining the indication for wavelength-specific filters, it is important to first differentiate the underlying cause of photophobia.

• Differential diagnosis of causative diseases: Photophobia is a symptom of many conditions such as migraine, blepharospasm, cone disorders, and post-traumatic syndrome. Identifying the cause leads to appropriate filter selection.
• Blink test: In blepharospasm, provocation is attempted using the blink test. There are three types: rapid blink test (quick blinking), gentle blink test (lightly closing), and forceful blink test (tightly closing), which evaluate the spasm provocation pattern.
• Pupillometry: This is a test that can quantitatively evaluate the function of ipRGCs.
• Chromatic pupillometry: This measures pupillary responses to light stimuli of different wavelengths and can selectively evaluate the melanopsin response.

Limitations of treatment indications

Wavelength-specific filters are expected to be effective for photophobia (afferent problems). On the other hand, they have not been shown to be effective for efferent problems such as convergence insufficiency and reduced reading speed. When these symptoms are predominant, alternative approaches such as vision therapy, rather than lens-based management, are necessary.

5. Standard Treatments

Use in Japan (Highest Priority)

Light-filtering glasses are listed as a treatment option for the following diseases.

• Blepharospasm (benign essential blepharospasm): Light-filtering glasses or clip-on glasses are listed as one of the treatment options. For patients with light-induced spasms or photophobia, trying light-filtering glasses may be considered.
• Cone dystrophy: In cases with severe photophobia, wearing light-filtering glasses is considered effective for symptom relief.

Filter Types and Recommendations by Disease

Recommended filters by disease and symptom are shown below.

Disease/Symptom	Recommended Filter	Evidence Trend
Migraine	FL-41 lens	Efficacy demonstrated
Benign essential blepharospasm	Light-filtering glasses, FL-41	Efficacy demonstrated
Post-concussion syndrome	FL-41 lenses	Subjective improvement (small trial)
Cone dystrophy	Red contact lenses	Most convincing improvement reported
Visual snow syndrome	FL-41 glasses	Reported efficacy for photophobia

FL-41 Lenses

• Tinted lenses that selectively block wavelengths around 480 nm²⁾.
• Efficacy has been shown in migraine and benign essential blepharospasm (BEB).
• Rose to amber lenses are common; the mechanism differs from sunglasses that simply reduce overall light.

Optical Notch Filters

• Thin-film coating on the lens surface allows more precise wavelength blocking than FL-41³⁾.
• Because it can target more specific wavelength bands, future clinical applications are expected.

Note: Variability in quality of FL-41 products

Cases have been reported where commercially available products labeled “FL-41” do not actually have the blocking characteristics of FL-41. The wavelength blocking characteristics may vary by product, so it is necessary to carefully verify the legitimacy of the supplier when purchasing. Also note that many studies have small sample sizes and the level of evidence is limited.

Q Which color lens is effective for which disease?

A

FL-41 lenses (blocking blue-green light around 480 nm) have shown effectiveness for migraine and benign essential blepharospasm. For cone disorders (cone photoreceptor dysfunction), red contact lenses show the most convincing improvement. For post-concussion syndrome, small-scale studies have reported improved subjective comfort with FL-41.

Q Are commercially available FL-41 lenses reliable?

A

Even if labeled as FL-41, some products do not have the specified blocking characteristics. It is important to verify that the supplier meets the official specifications before purchase. Consulting a specialist and selecting an appropriate product is recommended.

6. Pathophysiology and Detailed Mechanisms

Basics of ipRGC and Melanopsin

ipRGCs are a third type of photoreceptor cell that can detect light independently of rods and cones. They contain the melanopsin photopigment, with a peak absorption wavelength (λmax) of 482 nm.

• Giant ipRGCs: Approximately 3,000 exist, extending dendrites over a wide retinal area and expressing melanopsin.
• Diverse functions of ipRGCs: They form the main afferent pathway for the pupillary light reflex and are also involved in circadian rhythm, mood regulation, and visual image formation. It has been shown that light directly affects mood and learning via melanopsin-expressing neurons.

Integration of Pupillary Responses

Pupil diameter is determined by the additive integration of signals from the inner retina (ipRGC/melanopsin system) and the outer retina (rod and cone systems).

• Cone-mediated pupillary response: Short latency, fast contraction speed, and rapid recovery to baseline after light stimulation. Controls tonic contraction to contrast changes.
• Melanopsin-mediated pupillary response: Long latency, slow contraction speed, and sustained response. Sets the light-adapted pupil diameter during prolonged light exposure.
• Illuminance-dependent integration: Rod responses dominate during dark adaptation; melanopsin responses emerge during light adaptation as rods drop out; at higher illuminance, cone responses are added.
• Gain control: The gain control of pupillary regulation is presumed to reside in the Edinger-Westphal nucleus.

Aging and melanopsin function

Melanopsin function is relatively stable during the first 1–8 decades of life, after which a marked decline occurs. Since the melanopsin-mediated pupillary response remains relatively stable regardless of age, wavelength-specific filters can be expected to be effective even in older adults.

Theoretical basis of wavelength blocking

Blocking wavelengths around 481 nm reduces input to ipRGCs and suppresses signal transmission to nociceptive centers in the thalamus; this is the mechanism of action of FL-41 lenses and notch filters ^{2, 3)}. Even in cases where cones and rods are nonfunctional (e.g., retinal degenerative diseases), ipRGCs can still transmit light signals, so wavelength filters may contribute to photophobia reduction in these conditions as well.

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7. Latest research and future perspectives (reports at the research stage)

For Patients: Please Read Carefully

The following content is currently in the research or clinical trial stage and is not standard treatment available at general hospitals. It is reference information for professionals regarding future medical developments.

Optogenetics

Optogenetics research is underway to genetically restore lost photoreceptor function.

• Ectopic expression of melanopsin: Ectopic expression of melanopsin in RGCs has achieved improved pupillary reflex and restoration of light-dark discrimination in blind mice.
• Application of chimeric proteins: Research is progressing on expressing Opto-mGluR6 chimeras in ON bipolar cells to restore light responses in retinal degeneration models.
• Channelrhodopsin gene therapy: Clinical trials of gene therapy using channelrhodopsin are ongoing (e.g., NCT02556736).

Advances in ipRGC Research

In studies where melanopsin-directed immunotoxin was administered to rhesus macaques to selectively eliminate ipRGCs, significant attenuation of the pupillary reflex after ipRGC removal was confirmed, advancing the understanding of the role of ipRGCs in the pupillary light reflex.

Exploration of Non-Pharmacological Therapies

Repetitive transcranial magnetic stimulation (rTMS) is being investigated as a non-pharmacological therapy for photosensitivity in visual snow syndrome (VSS). Future research is awaited on the combined application of rTMS and wavelength-specific filters.

Challenges to Widespread Adoption

Currently, low availability in optical shops, insufficient awareness among doctors, and high costs for online purchases are barriers to the widespread adoption of wavelength-specific filters.

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8. References

1. Wilkins AJ, Wilkinson P. A tint to reduce eye-strain from fluorescent lighting? Preliminary observations. Ophthalmic & physiological optics : the journal of the British College of Ophthalmic Opticians (Optometrists). 1991;11(2):172-5. doi:10.1111/j.1475-1313.1991.tb00217.x. PMID:2062542.

2. Katz BJ, Digre KB. Diagnosis, pathophysiology, and treatment of photophobia. Surv Ophthalmol. 2016.

3. Hoggan RN, Subhash A, Blair S, Digre KB, Baggaley SK, Gordon J, et al. Thin-film optical notch filter spectacle coatings for the treatment of migraine and photophobia. Journal of clinical neuroscience : official journal of the Neurosurgical Society of Australasia. 2016;28:71-6. doi:10.1016/j.jocn.2015.09.024. PMID:26935748; PMCID:PMC5510464.