Purkinje images are reflections of light that occur within the human eye, named after Czech anatomist Jan Evangelista Purkinje, who first described them. These reflections, also known as Purkinje reflexes or Purkinje-Sanson images, offer insight into the eye’s optical properties. Observing these images helps understand how light interacts with the eye’s surfaces and forms the basis for tools used in vision science and ophthalmology.
The Four Reflections of the Eye
The eye produces four distinct Purkinje images, each originating from a specific surface. These images are formed as light passes through the eye, reflecting a small portion at each interface where the refractive index changes.
Purkinje Image I is the brightest and most easily observed reflection, originating from the outer (anterior) surface of the cornea. This image is erect and virtual, meaning it appears to be behind the reflecting surface and cannot be projected onto a screen. It is often referred to as the corneal reflection or “glint.”
Purkinje Image II is a much dimmer reflection, formed by light bouncing off the inner (posterior) surface of the cornea. This image is also erect and virtual, but its faintness makes it difficult to detect without specialized equipment. Its low visibility is due to the smaller difference in refractive index between the cornea and the aqueous humor behind it.
Purkinje Image III is produced by light reflecting from the outer (anterior) surface of the crystalline lens. This image is erect and virtual, similar to the first two, but generally dimmer than Purkinje Image I. It is also larger than the other images because of the curvature of the anterior lens surface.
Purkinje Image IV is unique among the four, as it is an inverted and real image, formed by reflection from the inner (posterior) surface of the crystalline lens. This image is the smallest and moves in the opposite direction to the light source. Its real nature means it can be projected onto a screen, distinguishing it from the other virtual images.
What Purkinje Images Reveal About the Eye
The distinct characteristics and movements of Purkinje images offer valuable information about the eye’s function and structure. Their practical applications extend across ophthalmology, optometry, and vision research.
One significant application is in eye tracking, where the positions of Purkinje images, particularly Image I (the corneal reflection) and Image IV, are used to monitor eye movements. By calculating the relative distance between these images, eye-tracking devices can determine gaze direction and track saccades (rapid eye movements) and fixations (when the eye holds steady on a point). This technology is widely used in psychological studies, human-computer interaction, and even in some clinical assessments.
Purkinje images also play a role in measuring corneal curvature, a technique known as keratometry. The reflection from the anterior corneal surface (Purkinje Image I) provides data used to fit contact lenses or plan refractive surgeries like LASIK. Precise measurements ensure optimal vision correction and patient comfort.
Observing Purkinje Images III and IV can help in assessing the crystalline lens. Changes in their appearance or position can indicate the presence of conditions like cataracts, which cloud the lens. These images can also be used to evaluate the centration and alignment of an intraocular lens after cataract surgery, ensuring it is properly positioned for clear vision.
The shifts in Purkinje images, especially Images III and IV, can reveal how the eye adjusts its focus for objects at different distances, a process called accommodation. As the lens changes shape to focus, the curvatures of its surfaces alter, causing corresponding changes in the reflections. This allows researchers to study the dynamics of the eye’s focusing mechanism.
Seeing Your Own Purkinje Images
It is possible to observe some of your own Purkinje images with a simple method, offering a direct experience of this optical phenomenon. While this personal observation is not a substitute for a professional eye examination, it can be an engaging demonstration. You might only perceive the brightest reflections, but it illustrates the concept.
To try this, find a dark room and use a small, bright light source, such as a penlight or the flashlight on a smartphone. Hold the light about 1 to 2 inches (2.5 to 5 cm) to the side of one eye, pointing it towards your eye but not directly into it. Slowly move the light slightly from side to side.
As you move the light, you may notice faint, shimmering reflections moving within your eye. The brightest and most distinct reflection you will likely see is Purkinje Image I, from the front surface of your cornea. You might also catch glimpses of the other, fainter reflections.