Slit lamp filters play a crucial role in modern eye examinations. These adjustments to the slit lamp illumination can increase your ability to assess the eye effectively. Each filter has its own purpose, helping practitioners assess, diagnose, and manage various eye conditions with precision. From improving contrast to highlighting fluorescein staining, they add incredible value to the slit lamp’s functionality.
Understanding these filters is essential for anyone wanting to make the most of this diagnostic tool. Whether you’re using the green-tinted red-free filter to examine retinal vessels or the cobalt blue filter to detect corneal damage, each has its place in clinical practice.
In this article, we’ll explore the types of slit lamp filters available and their practical applications. By the end, you’ll have a clearer view of how they can elevate the quality of your eye examinations.
Where to Find the Slit Lamp Filters
The location of the slit lamp filters can vary depending on which of the two types of slit lamp that you may have.
On Keeler slit lamps, the slit lamp filters are typically located on a rotating dial below the illumination source. This dial allows you to switch between different filters, such as the red-free, cobalt blue, and neutral density options. The design ensures quick and easy access during examinations.
For Haag-Streit slit lamps, the slit lamp filters are integrated into the illumination barrel at the top of the slit lamp. They are often found directly above the beam height dial.
Red-Free Filter
Colour: Green
Wavelength: 540 nm to 570 nm1
Icon: Green-Filled Circle / Green Line
The red-free filter is one of the most useful slit lamp filters for detailed ocular examinations. This green-tinted filter works by blocking red light, enhancing the contrast between red- and not-red structures and findings.
It’s especially effective for assessing the retina, as it creates a stark contrast between blood vessels and surrounding tissue1,2,3,4, as well as highlighting the retinal nerve fibre layer1.
In practice, the red-free filter is invaluable for detecting retinal hemorrhages or loss of the nerve fiber layer. It can also help distinguish between a retinal lesion and a choroidal nevus. These subtle but important distinctions are vital for accurate diagnosis and treatment planning.
Using this filter is straightforward. Once engaged, the slit lamp filter immediately transforms the view, making haemorrhages and blood vessels stand out more clearly. This allows optometrists to detect even minor changes that may indicate an underlying issue. The red-free filter is often overlooked in clinical practice, but optometrists should be reminded the filter exists and used to assist in their examinations.
Cobalt Blue Filter
Colour: Blue
Wavelength: 465 nm to 490 nm3
Icon: Blue-Filled Circle / Blue Line
The cobalt blue filter is one of the most visually distinct slit lamp filters. The cobalt blue filter works hand-in-hand with fluorescein dye to reveal hidden details on the eye’s surface2. Once the dye is applied, switching to this filter makes the dye emit a bright green glow that highlights corneal irregularities2,3,4. This makes it a go-to tool for spotting abrasions, ulcers, or even tiny foreign bodies that are easy to miss.
I would like to highlight a little quirk with the cobalt blue filter on Haag-Streit type slit lamps; sometimes the filter isn’t selectable on the switch with the other slit lamp filters. If it appears to be missing, try rotating the beam height dial fully clockwise or fully anti-clockwise to see if this activates the filter. You will eventually get to know where to access the complete range of slit lamp filters for your slit lamp – but do check this if you are initially struggling to find it!
Optometrists often rely on this filter when measuring intraocular pressure by Goldmann applanation tonometry (GAT)2,4, assessing dry eye4 or fitting contact lenses4. It’s particularly helpful for measuring tear film break-up time or pinpointing areas of dryness on the ocular surface. If you’ve ever discovered a subtle corneal disruption thanks to that vibrant fluorescence, you’ll know why this filter is indispensable.
Neutral Density and Grey Slit Lamp Filters
Colour: White light of reduced intensity
Wavelength: N/A – white light full spectrum
Icon: Neutral density is a circle composed of a hashed line; Grey filter is a circle composed of a solid line.
These two slit lamp filters are there to help reduce the illumination, which in turn may help assess the patients eye. There is no wavelength information as it uses white light under reduced intensity.
Neutral Density Filter. The neutral density filter is a staple among slit lamp filters. Its grey tint reduces the light’s intensity without altering the colours of ocular structures4,5. This is particularly helpful during examinations involving photophobic patients or when assessing light-sensitive areas of the eye. It lets practitioners examine delicate structures, such as the macula, while keeping the patient comfortable.
This filter is also invaluable in reducing the strain on both patient and practitioner during longer sessions. By toning down the brightness, it ensures a detailed and thorough examination without overwhelming the eye5.
Grey Filter. The grey filter is another indispensable tool in the optometrist’s arsenal of slit lamp filters. It moderates light intensity effectively, improving both safety and clarity during extended use. The reduced glare helps practitioners focus on fine ocular details without distraction. However, it does not maintain accurate colour rendering.
This filter is especially useful when examining patients who experience discomfort under bright lights. Its ability to soften the light ensures a smooth and effective procedure.
These two slit lamp filters highlight the importance of light management during the ophthalmic health examination, ensuring both effectiveness and patient care remain a priority.
Additional Slit Lamp Filters
Whilst most optometrists think that the slit lamp filters are all located in the same place, there are some additional slit lamp filters to be aware of. These additional slit lamp filters will aid in your assessment of the eye. Please note, these slit lamp filters appear on most modern models, but please check your slit lamp’s manufacturer instructions for more details.
Heat-Absorbing Filter
Icon: A circle filled in with lines (Note: Not an option on many modern slit lamps)
The heat-absorbing filter is a vital feature of many modern slit lamps. Although visually clear, it performs an important role in safeguarding both the patient and practitioner. By absorbing infrared light, it prevents heat build-up during longer examinations.
This filter is especially useful when the slit lamp’s light source is used at high intensity or for extended periods. It ensures the procedure remains safe, avoiding any discomfort caused by heat. Slit lamp filters like this one show how design adaptations protect patient health while enhancing the user experience.
The heat-absorbing filter isn’t a filter that optometrists actively notice, as it’s integrated into most slit lamp devices. However, its presence highlights how innovation in slit lamp filters addresses the challenges of modern eye care.
Diffusing Filter
This filter is located between the slit lamp illumination mirror and the patient’s eye5. There is no dial or switch to push, but you will need to move the filter between the illumination mirror and the patient to diffuse the light.
The diffusing filter does exactly what its name suggests—spreads light evenly across the field of view4,5. Instead of a focused beam, it creates a softer, more uniform illumination. This helps reduce shadows and glare, giving a clearer overall view of the eye’s anterior segment.
This filter is particularly handy during initial examinations when you need a broad overview of the eye’s surface2,4. It’s also useful for assessing general conditions without honing in on specific details. With its frosted or opaque appearance, the diffusing filter ensures the light isn’t too harsh on sensitive eyes.
By softening the light, this filter adds comfort for patients and convenience for practitioners. It’s not always needed for detailed examinations, but when you want a smoother, wider view, the diffusing filter is the perfect choice among slit lamp filters. Its simplicity is what makes it so effective.
Wratten #12 Filter
The Wratten filter (Kodak Wratten #126), yellow in colour, serves a unique purpose in enhancing diagnostic clarity. Its role is to block blue light reflected from the patient’s eye from reaching the examiner’s eyes through the oculars. This boosts the contrast of the green fluorescence. This results in sharper and more defined images during slit lamp examinations6.
Many modern slit lamps come with a built-in Wratten filter6, making it easy to switch between settings. However, if your device lacks this feature, don’t worry—you can achieve the same effect with a handheld filter. Simply hold it in front of the oculars after using the cobalt blue filter to highlight fluorescein staining.
One key tip: If your slit lamp has a built-in Wratten filter, remember to remove it after the fluorescein assessment. Forgetting to do so will leave everything looking yellow, which can obscure your view for the remainder of the examination. This small detail can make a big difference in your workflow. I mention this because I have heard many a rep tell me they get many a phonecall saying that their slit lamp view is yellow!
The Wratten #12 filter is particularly useful in assessing corneal staining, tear film quality, and dry eye conditions. Whether integrated or handheld, it’s one of the slit lamp filters that every practitioner should have in their toolkit for improving diagnostic precision.
Summary on Slit Lamp Filters
Slit lamp filters are an essential part of any optometrist’s toolkit. Each one serves a specific purpose, whether it’s improving contrast, highlighting fluorescein staining, or softening light for a clearer view. By understanding how and when to use them, you can ensure more accurate diagnoses and a better experience for your patients.
From the red-free filter’s ability to enhance retinal details to the practical comfort offered by the neutral density or grey filters, these tools make a tangible difference. They’re key to delivering high-quality eye care. Next time you’re at the slit lamp, take a moment to consider how these slit lamp filters can elevate the standard of your examination.
With the right knowledge, these slit lamp filters become more than just accessories—they’re a way to provide clearer, more effective care. It’s all about making the most of the tools at your disposal. So don’t forget about them!
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Skill Activity
Slit Lamp Filters Quiz
In this skill activity, we will cover the slit lamp filters. Please do your best to answer the questions to see how much you know about the slit lamp filters. Share in the comment about how you get on!
Further Reading on Slit Lamp Filters
- Leburg S (2021). Rediscover your red-free. Review of Optometry [Online.] Available at: https://www.reviewofoptometry.com/article/rediscover-your-redfree [Accessed: March 11th 2025].
- Siddique A (2022). The slit lamp examination: an introduction. Journal of the Foundations of Ophthalmology [Online.] Available at: https://jfophth.com/the-slit-lamp-examination-an-introduction/ [Accessed: 11th March 2025].
- Labib LA (2021). Reach for the dye. Review of Optometry [Online.] Available at: https://www.reviewofoptometry.com/article/reach-for-the-dye [Accessed: March 11th 2025].
- Nanda A, and Kotecha A (2014). How to conduct a slit lamp examination. British Undergraduate Journal of Ophthalmology 2(1): 1-6.
- DelMonte DW, Brandner D, Yamanuha J, and Montezuma SR (2024). Slit lamp examination. American Academy of Ophthalmology [Online.] Available at: https://eyewiki.org/Slit_Lamp_Examination [Accessed: March 11th 2025].
- Srinivas SP, and Rao S (2023). Ocular surface staining: current concepts and techniques. Indian Journal of Ophthalmology 71(4): 1080-1089.
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