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What is the Purkinje Image?

The so-called “Purkinje image” is the reflection of the object that you can see in the eye of a person. Purkinje –Sanson images and Purkinje reflexes are also the common names that are often associated with the word ‘‘Purkinje Image.” There are four or less Purkinje images that are noticeable. The P1 or the number one Purkinje image is the image that you can see on the outer area of the person’s cornea. The P2 or the number 2 Purkinje image is the picture that you can see inside of the cornea’s surface. The P3 or the number 3 Purkinje image is the picture that you can see outside or the anterior of the lens surface. And the P4 or the number 4 Purkinje image is the picture that you can see inside or in the posterior of the lens surface. P4 Purkinje image is different from the first three Purkinje images because it will give you inverted images.

The name Purkinje –Sanson images derived from the name of Jan Evangelista Purkyne, a Czech anatomist, from the year of 1787 to 1869 and also from the name of Louis Joseph Sanson, who is a French physician, from the year of 1790 to 1841.

The fourth and third type of Purkinje image is noticeable from the eye of a person. The light that will reflect from the lens surface will also reflect on the person’s eye stern cornea’s surface. The images like these are called the “entoptic phenomena.”

The number 1 and number 4 of the Purkinje image is being used by different eye tracker devices, which is the type of digital item that is being used in measuring the person’s eyes position and movement. Glint is the name of the digital device that was being used in measuring the Number 1 Purkinje image.

Brightens of Purkinje Images

For you or for your optometrist to measure the brightness of your Purkinje images, it is preferable for them or for you to use the so-called “Fresnel’s equation.” The Fresnel’s equation will be like this:

intensity or the Brightness = (n1 – n) 2/ (n1 + n) 2

wherein the n1 and then in the Fresnel’s equation refers to the “refractive indices” after and before a thing reflects the surface. The P1 Purkinje image is the image that has the brightest color compared to the remaining three Purkinje images, and then the P4 and P3 Purkinje images have a similar brightness and the P2. Both of these types of Purkinje images can evaluate the separations and curvatures of the patient’s eye surface.

Purkinje Images and Eye Tracking

We all know that the executions and the uses of eye tracking are increasing as time passes by. Purkinje images are the methods that are being used by the professionals to track the movement and the position of the person’s eyes. Knowing the uses and how the Purkinje images are being used is very important for both the optometrists and the patient.

Eye Tracking and Purkinje Images

There are lots of eye tracking items that have been developed and used by many eye care professionals to measure the position and the movement of the person’s eyes. The Purkinje images are one of these eye-tracking methods that can be used to detect both the movement and the position of the patient’s eye. The optometrists commonly use both the P4 and the P1 of the Purkinje images in detecting the eye position and movement. P3 Purkinje image can also be used, but occasionally. The method of using the P4 and P3 Purkinje images were being used by optometrists to track the eye position and movement through measuring the space between the P1 and P4 Purkinje images. This method is according to Lewis in the year of 1977. This method was used because the spaces between the P1 and P4 images are constant when the patient takes a glare at a similar optical axis plane. For instance, the plane surface wherein the patient’s eye would glare at will give a similar measurement for the spaces of P1 and P4 Purkinje images. If the space between the two Purkinje images has changed, it only means that the person’s eye takes a look at the different optical axis surface.

This kind of eye tracking method will only work at the 2D because it will enable you to determine if the person’s eye is only looking at the same plane optical axis surface through the Purkinje images distance measurement. This may not give you useful and reliable solutions for the patient’s present eye tracking because this kind of execution is difficult and large.

Even though some of the present eye tracking executions will still vary upon the Purkinje images detection, many optometrists have switched to use the video eye tracking with the bright and dark detection of a person’s pupil. Through using the video eye tracking method, it will give both the patient and the optometrist to have the photos of the patient’s pupils that will either appear on the dark and bright side that will use the location changes of the person’s eye to distinguish the way where the eye is looking at. This video eye tracking method has also ’it’s limit. It is when the pupil detection is lost, that can occur occasionally. Thus the entire video eye tracking system must attempt and reset the pupil detection again.


The Purkinje images eye detection method is very useful to determine the complexion of the patient’s eyes and also to measure the other data about the person’s eye. There are lots of tracking methods that were successful through the use of the Purkinje images. But some latest methods were developed for eye tracking. It only means that both the patients and the eye care professionals are in need to have clear knowledge and ideas about the uses and the works of the Purkinje images.