Fluorescein that has been instilled into the eye is best illuminated by using:

Fluorescein is a dye commonly used in diagnostic procedures to examine the eye, particularly in assessing the tear film and identifying corneal abrasions or foreign bodies. When fluorescein is instilled into the eye, it absorbs light in the blue spectrum and emits a bright yellow-green fluorescence. Therefore, the best way to visualize fluorescein-stained areas is through the use of a cobalt blue filter.

The cobalt blue filter allows for the optimal wavelength of light (around 450 nm) to illuminate the fluorescein, enhancing the contrast and making the areas where the dye has accumulated appear more prominent. This improves the ability to assess the integrity of the corneal surface and detect any abnormalities.

While other methods, such as sclerotic scatter or using a green filter, could provide some illumination or help in observing certain characteristics of the eye, they do not specifically enhance the visualization of fluorescein. A pen light may be useful for general inspection but lacks the capability to highlight fluorescein fluorescence effectively. Thus, the cobalt blue filter is the best choice for illuminating fluorescein in the eye, allowing for clearer visualization during ocular examinations.