Streak Retinoscopy – Optics, Computer simulation, Observations and Findings

Ai-Hou Wang, M.D., Ph.D.

To write a computer program to simulate the pupillary reflex observed in retinoscopy, the optics of retinoscopy were analyzed from scratch.

The results of the computer simulation are as follows:

Optical Analysis of Streak Retinoscopy

Optical analysis consists of two parts: the observation system and the illumination system.

The observation system calculates the area of the retina visible through the pupil.

The illumination system calculates the illuminated area of the retina.

The intersection of these two systems is the visible bright area.

The proportion of this bright area within the visible retinal area is the proportion of light reflex seen within the pupil.

Figure 1

The independent variables for calculating the observation system are:

refractive power of the eye(R)

pupil diameter (Pu)

test distance (L)

Figure 2

The independent variables used to calculate the illumination system are:

Vergence of the illumination light (vgnc)

Width of the light entering the pupil (lip)

Figure 3

Calculate the retinal area seen through the pupil

Figure 4

Calculate the distance to the focal plane

Figure 5

Regardless of whether the illumination light is converging or diverging, its midline must pass through the intersection of ‘the conjugate plane of the observation system’ and the central axis.

This is a crucial step in the optical calculations combining the observation and illumination systems.

Figure 6

The intersection of the illuminated retinal area (iar) and the retinal area seen through the pupil (rar) is the reflex portion of the pupil as seen during retinoscopy.

Figure 7

Calculate the width of the illumination light on the pupillary plane (face).

Figure 8

Calculate the distance between the focal point of the illumination beam and the central axis.

Figure 9

Calculate the distance between the leading edge of the retinal illumination zone and the central axis.

Figure 10

Calculate the distance between the trailing edge of the retinal illumination zone and the central axis.

Figure 11

Computer Simulation of Retinoscopy

Computer Simulation of Retinoscopy Hyperlink

Based on geometric optics, we wrote a dynamic simulation of retinoscopy on personal computer. The variable parameters are (1) test distance, (2) pupil diameter, (3) the vergence of light source and (4) patient's refractive power.

This program runs only on Windows systems. Doesn’t work on Android or iOS.

Clicking the online link will immediately download the program "StreakRetinoscopySimulation.exe".

Run the application "StreakRetinoscopySimulation.exe".

Select "More Information" and then "Run Anyway".

Key Instructions:

<F1> Key Instructions Screen

<Right> Fast < > > Slow Move the band light to right

<Left> Fast < < > Slow Move the band light to left

<Down> Center the moving band light

<2> Increase test distance <1> Decrease test distance

<W> Increase pupil diameter <Q> Decrease pupil diameter

<S> Increase divergence of the light source <A> Decrease divergence of light source

<X> Increase patient’s refractive power <Z> Decrease patient’s refractive power

<O> Increase the initial value of the band light entering the pupil <I> Decrease the initial value of the band light entering the pupil

(Distance of the right border of the band light entering the left border of the pupil)

<Esc> Terminate the program.

Parallel Illuminaion (0D)

(1)+8D (2)+5D (3)+1D

(4)0D (5)-0.75D (6)-3D

(7)-3.75D apparent against-movement (8)-5D (9)-8D apparent with-movement

(1)+8D (2)+5D (3)+1D

(4)0D (5)-0.75D (6)-3D

(7)-3.75D (8)-5D (9)-8D

Divergent Illumination (-2D)

(1)+8D (2)+5D (3)+1D

(4)0D (5)-0.75D (6)-3D

(7)-3.75D apparent against-movement (8)-5D (9)-8D apparent with-movement

(1)+8D (2)+5D (3)+1D

(4)0D (5)-0.75D (6)-3D

(7)-3.75D (8)-5D (9)-8D

Convergent Illumination (+4D)

(1)-12D (2)-9D (3)-5D

(4)-4D (5)-3.25D (6)-1D

(7)-0.25D apparent against-movement (8)+1D (9)+4D apparent with-movement

(1)-12D (2)-9D (3)-5D

(4)-4D (5)-3.25D (6)-1D

(7)-0.25D (8)+1D (9)+4D

New Observations on Streak Retinoscopy

Observation (1) – The proportions of the schematic diagrams drawn in general textbooks are incorrect

The corneal diameter is about 12mm, and the width of the parallel light source of the retinocope is about 10mm. It should not be as thin as shown in the diagram. If it is a divergent light source, the width of the band light on the face should be even wider.

The width of the streak light source and its proportions to the corneal diameter and pupil diameters are depicted based on actual dimensions. The left figure shows a parallel light source, and the right figure shows a diverging light source in proportion to the actual corneal size.

Observation (2) – The against movement never displays a "band-like" reflex.

Most textbooks describe both with and against movement as thin "band-like" reflex, much thinner than the pupil's diameter. However, in reality, against movement never displaces a "band-like" reflex.

Let's first look at the ‘with movement’ in the computer simulation –

Parallel light source, 50cm test distance, -0.75D refraction will display ‘with movement’.

When the illuminating light enters from the left border of the pupil, the left border of the pupil lights up first, and the reflex also moves to the right in the same direction.

When the illuminating light reaches the pupil midline, a band reflex appears in the pupil.

The illuminating light continues to move to the right, the reflex also moves to the right, gradually exits the pupil at the right border.

Parallel light source, 50cm test distance, -3D refraction will display ‘against movement’.

When the illuminating light enters from the left border of the pupil, the right border of the pupil lights up first, and the reflex moves to the left in opposite direction.

When the illuminating light moves gradually into the pupil, the whole pupil lights up, without ever showing a band reflex.

The illuminating light continues to move to the right, and the reflex goes to the left half of the pupil.

The illuminating light gradually moves out from the right border of the pupil, and the final reflex moves out from the left border of the pupil.

In a dynamic state, it appears as if a ‘band-shaped reflex, wider than the pupil’ is moving from right to left, entering from the right border and exiting from the left border of the pupil.

However, at any instant, you can only see either the left or right edge of the reflex; you never see both edges simultaneously.

In other words, you don't actually see a true ‘band-shaped’ reflex within the pupil.

The retinoscopy technique in the AAO BCSC Clinical Optics book previously depicted the against movement reflex as a ‘band-shaped’ reflex. In 2014-2015 edition, Chapter 3 Clinical Refraction, p. 95, it has been corrected to show only a single edge of the pupillary reflex.

New Discoveries in Streak Retinoscopy

The optical principles of retinoscopy in textbooks state that ‘when the patient's far point falls between the patient and the examiner, the pupil will exhibit against-movement reflex.’ This is indeed the case for low myopia, such as -2.75D. However, computer simulations of the pupillary reflexes in higher myopia reveal that the reality is not so simple; not all reflexes are against-movement.

Discovery (1) – The pupillary reflex in moderate myopia is ‘apparent against-movement’.

The illustration shows a -3.75D myopia, retinoscopy performed at 50 cm using a parallel light source. The illuminating light enters the pupil from the left border. Instead of the pupil's border lighting up first, an area ‘inside’ the pupil, slightly to the right, lights up first.

As the illuminating light enters the pupil further, the reflex widens quickly to touch the pupil's right border.

After that, only the left edge of the reflex is visible in the pupil, moving leftward. It appears similar to an against-movement reflex, which we name as ‘apparent against-movement reflex’.

Discovery (2) – The pupillary reflex in high myopia is "apparent with-movement".

The illustration shows a -8D myopia. The illuminating light enters the pupil from the left border, and an area inside the pupil, slightly to the left, lights up first. As the illuminating light moves into the pupil, the reflex widens quickly to touch the left border of the pupil.

Afterward, only the right edge of the reflex can be seen moving to rightward in the pupil, which looks like a with-movement reflex. We name it as ‘apparent with-movement reflex’.

From moderate myopia to high myopia, the ‘apparent against-movement reflex’ gradually changes to ‘apparent with-movement reflex’. At approximately -5D, when the illuminating light enters the pupil from the border, the pupillary reflex will first light up in the very center of the pupil!

New Concepts in Retinoscopy

(1) From the above findings, we know that when you see a dark, with-movement reflex you should not instantly assume it to be high hyperopia; it could possibly be an apparent with-movement of high myopia.

Of course, this problem could be easily solved. Simply add a negative lens (~-4D) or a positive lens (~+4D) to over-refract, and you can immediately tell whether the patient is high hyperopia or high myopia.

Of course, this new discovery does not affect the neutral point reading of retinoscopy.

(2) If retinoscopy is performed using a convergent light source, the with-movement / against movement will be the opposite of those using a divergent/parallel light source.

What will our new observations and findings look like when using a convergent illumination?

1 – The against-movement similarly will not show a ‘band-shaped’ reflex.

Myopia -1D, retinoscopy at 50 cm using convergent light source (+4D).

2 – As hyperopia gradually increases, the pupillary reflex changes from against-movement to ‘apparent against-movement’, and then gradually to ‘apparent with-movement’.

Retinoscopy at 50 cm using convergent light source (+4D) for myopia -0.25D (apparent against-movement), hyperopia +1D, and hyperopia +4D (apparent with-movement).