Welcome to question of the day #15

Welcome to question of the day #15

Why does fluorescein fluoresce?

Many times during my time as a student at optometry school I was told to do something without the underlying science being explained to me. Sometimes I was inquisitive and asked. Sometimes I got a response. One procedure I carried out routinely without knowing the underlying science was an examination of the cornea with fluorescein and cobalt blue light. It was many years later that I stumbled across the underlying science.

The fluorescein we use, that is impregnated onto single-use lint free paper strips, is the sodium salt of fluorescein. It is used by optometrists to determine the integrity of the cornea, conjunctiva and tear film and in the evaluation of rigid gas permeable contact lens fitting.

The paper is wetted with sterile saline and applied to the bulbar surface of the conjunctiva or lower fornix releasing fluorescein into the tear film and onto the surface of the eye. On the paper fluorescein is orange. In the eye and when illuminated with cobalt blue light it is green/yellow. It is called cobalt blue light because it has the same colour as cobalt (II) oxide-aluminium oxide which is a blue pigment. This is used as a colouring agent in ceramics, jewellery, paint and transparent glass.

Cobalt blue light provides a means of exciting sodium fluorescein when in its orange form. The maximum transmission through the cobalt blue filter is in the region of 390nm to 410nm. Fluorescein maximally absorbs light of wavelengths between 485nm and 500nm. This causes the orange fluorescein to fluoresce and emit a green/yellow light of maximum intensity at between 525nm and 530nm. Fluorescein atoms absorb the cobalt blue wavelengths and this causes electrons to move to higher orbitals. No sooner have they reached these higher orbitals they drop down to lower orbits and this causes the emission of green/yellow light. This is the process of fluorescence.

From the wavelength figures above you will notice that the light that is usually used in slit lamps with a blue filter is not at the best wavelength to give maximum absorption by fluorescein and therefore fluorescence of the dye. Some slit lamps contain filters producing more optimum wavelengths of blue light. The view of fluorescein can be further enhanced by using a yellow Wratten 12 filter to cut-out any blue light that is reflected back in to the slit lamp from the eye.