/ A practical guide to slit lamp filters
13.01. Tue
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A practical guide to slit lamp filters

While most slit lamp examinations are conducted under standard white light illumination, offering a true-to-life view of the eye in its natural state, additional filters can significantly enhance a clinician’s ability to visualise structures and detect pathology. Filters provide more control over the quality, intensity, and wavelength of light used, each serving a specific clinical function depending on the structure being examined or the condition suspected.

On Haag-Streit slit lamps, filters can be applied at three different locations, providing clinicians with flexibility in their examination technique. These filters either block, diffuse, or selectively alter the light beam to improve visibility and contrast for particular diagnostic tasks.

The video below shows how to easily change the filters on a Haag-Streit slit lamp.

 
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When to use a diffuser: broad illumination for general assessment

The diffusor is a removable filter located in front of the slit lamp’s illumination mirror. To activate it, the user simply flips the diffusor into place. This filter spreads the slit beam to provide a soft, generalised illumination across the entire field of view. When used with low magnification, it allows for an overall assessment of the eye’s anatomy.

The diffusor can serve as a practical substitute for a dedicated background illuminator

However, this broader illumination comes at the expense of fine structural detail. The diffusor softens depth cues and reduces the visibility of curvature and subtle texture. Despite this trade-off, it has the advantage of being compatible with other filters, allowing clinicians to combine it with additional illumination modes for a more comprehensive view.

 

 

When to use a neutral density filter: comfort & clarity for sensitive examinations

The neutral density filter is engaged by turning the filter lever to a designated position on the Haag-Streit slit lamp. This filter works by reducing the intensity of the slit beam. It does not alter the colour of the light but simply decreases its brightness.

This makes the neutral density filter especially useful in cases where the patient is photosensitive, as it enables a more comfortable examination without compromising visibility. It is also useful when examining the posterior segment, where excessive brightness can produce excessive glare and reflections from the fundoscopy lens for the observer, or cause discomfort to the patient.

 

When to use a cobalt blue filter: visualising fluorescein & diagnosing surface pathology

The cobalt blue filter emits light in the blue spectrum, specifically between 465 and 490 nanometres, which is ideal for exciting sodium fluorescein dye. This filter should not be confused with the green or red-free filter, which serves a different purpose. The blue filter is not located on the filter wheel. To engage the blue filter, the slit lamp height wheel should be rotated anti-clockwise until the blue filter icon is seen.

Benefits of a blue filter with fluorescein

When used in conjunction with fluorescein, the cobalt blue filter makes the dye fluoresce, allowing clinicians to visualise the tear film and ocular surface more clearly. This is beneficial in the diagnosis and monitoring of conditions such as dry eye disease, corneal ulcers, and ocular perforations. Additionally, it is essential for performing applanation tonometry, where the glowing fluorescein mires must be aligned to accurately measure intraocular pressure.

When to use a yellow filter: enhancing fluorescein imaging without altering illumination

The yellow filter is an accessory component situated next to the magnification. It is engaged using the same filter lever system. The yellow filter is a modular accessory, which can be added to the BQ 900, BP 900, and BX 900 Haag-Streit slit lamps. A separate plunger pulls up to encage the filter. This filter acts as a barrier filter rather than altering the illumination itself. Instead, it changes what the clinician sees through the eyepieces by blocking blue wavelengths from entering the observer's eye.

Used in combination with the cobalt blue filter, the yellow filter enhances the contrast of the fluorescein signal, making subtle surface details more pronounced. This is particularly valuable during rigid gas permeable (RGP) contact lens fittings, where assessment of the post-lens tear film and ensuring proper edge lift are crucial for comfort and ocular health.

When to use a red-free (green) filter: enhancing vascular contrast & localising lesions

Often underutilised, the red-free filter, also referred to as the green filter, is another tool available via the slit lamp’s filter lever. This filter selectively blocks the longer wavelengths of light at the red end of the visible spectrum. As a result, red-hued structures, such as blood vessels, appear darker and stand out more distinctly, enhancing contrast.

This improved visibility makes the red-free filter particularly useful for examining the vasculature structure in both the anterior and posterior segments of the eye. It enables the detection of subtle vascular changes that may go unnoticed under standard white light.

Furthermore, the filter can assist in localising retinal lesions. Green light does not penetrate the choroid, so lesions located there, such as choroidal nevi, become less visible or disappear under green illumination. In contrast, lesions like CHRPE (congenital hypertrophy of the retinal pigment epithelium), which reside in more superficial layers, remain visible.

Additional filters: opening for filter of choice 

The final position on the filter wheel is empty. This allows for customisation of the slit lamp. If required, an additional filter can be fitted here.

 

Conclusion

While white light remains the foundation of slit lamp examination, filters unlock a higher level of precision and diagnostic capability. From increasing patient comfort with neutral density filters to enhancing contrast for vascular assessment and fluorescein imaging, each filter plays a specific role. By understanding their uses and limitations, clinicians can refine their examinations, leading to more accurate diagnoses and better patient outcomes.