Introduction Copy

Introduction

Visual acuity testing is an important part of the fitness assessment of risk exposed workers in the workplace. The purpose of the visual acuity assessment is to identify visual ability, any result outside of the normal range can impact safety and efficient productivity. Visual acuity testing is a means to prevent occupational eye injuries and accidents.

Vision screening in the working place is done to provide evidence that the person meets the job requirements in respect of vision and is fit for duty in terms of the visual requirements. The required level of vision required in each area of work or position varies according to the job a person does. For example, people who drive and operate moving equipment have different visual requirements to someone working on a computer all day.

It is important that the technician who performs the vision screening is competent to test vision correctly and acutely. A competent person must perform all vision screening assessments. The South African Qualifications Authority (SAQA) states that a competent person is a person who has a vision screening certificate based on the requirements set out in Unit Standard (US) 116505 on vision screening in the workplace⁶, this requires the technician to check the equipment, test the eyes with a vision screener (manually or automated), keep a record of the findings and report on the vision screening test results.

During visual acuity testing workers are screened for:

Distance and near vision
Colour vision
Visual fields
Depth perception

Workers who wear glasses or contact lenses are examined with and without correction. Visual acuity is affected by many factors, such as near-sightedness, farsightedness, astigmatism, and a range of eye diseases.

Near-sightedness, also known as short-sightedness or myopia, is an eye disorder where light focuses in front of, instead of on, the retina. In a near-sighted eye, the eyeball is elongated or stretched, creating a longer distance between the cornea and the retina (the “front” and the “back” of the eye) and causing the cornea to assume a different shape. This causes blurriness in vision when trying to focus on objects too far away. It is a common vision condition in which objects nearby are clear and objects farther away are blurred. This can be problematic if the worker is a forklift driver for example.

Figure 8.1: A comparison of the normal eye versus the near-sighted eye²

Farsightedness is a common condition affecting the eye in which the worker can see distant objects clearly, but nearby are blurred. Worldwide, an estimated 1.3 billion people have it. It’s expected to increase to 2.1 billion by 2020¹⁰. There are two kinds of farsightedness, presbyopia, and hyperopia Both conditions affect a worker’s ability to perform detailed close work or extensive reading. Presbyopia is age-related and is the decreased ability to see nearby objects, especially in low light. Presbyopia usually starts around the age of 40 as the lens of the eye begins to harden and become less flexible and therefore no longer changes its shape to focus on images up close. Hyperopia is often hereditary, the eye is slightly shorter than normal. Farsightedness is generally corrected with glasses or contact lenses¹⁰.

Figure 8.2: A comparison of the normal eye versus the far-sighted eye³

Colour vision deficiency is defined as the inability to distinguish certain colours. This usually happens between greens and reds, and occasionally blues. Cone cells in the retina detect colour, there are three types of cones red, green, and blue. Colour vision deficiency occurs when one or more of the colour cone cells are absent or detect a different colour than normal. Severe colour vision deficiency occurs when all three cone cells are absent. There are different degrees of colour vision deficiency. Some people with mild colour deficiencies can see colours normally in good light but have difficulty in dim light. Others cannot distinguish certain colours in any light. The most severe form of colour vision deficiency, in which everything is seen in shades of grey, this is uncommon.

Colour vision deficiency usually affects both eyes equally and remains stable throughout life. Colour vision deficiency usually occurs from birth but can occur later in life which is serious and should be referred for assessment and management. Three types of colour vision deficiencies exist. Red-green deficiency where people cannot distinguish certain shades of red and green. This is the most commonly inherited type. Blue-yellow deficiency is rare, and the person has difficulty in distinguishing between blue and green, and yellow may appear as pale grey or purple.


	●	Deuteranopes – do not see green (absolute – Deuteranopia; partial – Deuteranomalia)22
	●	Protanopes: do not see red (absolute – Protanopia; partial – protanomalia)22
	●	Tritanopes: do not see either blue or yellow22

The visual field is what the eye sees when the head and eyes are fixed on a central point directly in front²³. The range of what can be seen above, below, and on either side of while gazing straight ahead is the visual field. It includes central and peripheral vision and is usually measured in degrees of visual angle. Most essential job functions need normal visual fields. Abnormal visual fields are often caused by conditions like glaucoma, diabetes, high blood pressure, and stroke.

Depth perception refers to the ability to see objects in three dimensions (3-D), length, width, and depth and to judge movement and distance. Stereopsis testing is important in identifying amblyopia, strabismus, suppression, and stereopsis^23,24.

Far Visual Acuity Testing

Far visual acuity is tested manually with a Snellen or a tumbling E chart or using an electronic visual acuity tester such are a Keystone, Titmus or another automated vision tester. Any abnormalities identified in automated testing indicate that a manual test should be done.


Snellen Chart testing of Far Visual Acuity
●	Each eye must be tested separately, and both eyes together.
●	Test without correction and repeat with glasses or contact lenses after the uncorrected test
●	Explain test to the worker
	o	Stand at the designated line.
	o	Cover 1 eye at a time but don’t apply pressure to the eye being covered.
	o	The test starts on the right side or with the better eye first.
	o	The assessment is first done without glasses and then repeated with glasses.
	o	Each eye is tested separately and both eyes together.
	o	Don’t lean forward, squint, or peep during the test.
	o	If uncertain or if anything is unclear the test can be stopped.


●	Ensure the worker is standing on the line that has been measured out during preparation.
●	Remember if the worker is unable to read, use the tumbling E chart.
●	The worker must cover the right eye first or if the better eye is known start with the better eye.
●	The technician stands at the chart, with a pointer and points to the images one by one, starting at the largest letter on the chart, this is to ensure the worker has understood the instructions.
●	The technician then points to the images one by one at the 20/20 or 6/6 line.
●	If the worker correctly identifies all 8 letters correctly on the 20/20 or 6/6 line, not further testing is required as this is normal vision. Change eyes and repeat ending with testing binocular vision – both eye are used to see the chart,
●	If the worker gets 2 letters incorrect on the 20/20 or 6/6 line, move one line up to the 20/25 or 6/9 line and repeat the procedure. Continue moving up the lines until the worker is able to correctly identify at least 50% of all the letters on each line.
●	Then change eyes and repeat ending with testing binocular vision – both eye are used to see the chart.
●	The worker calls out the letters the technician has pointed to.
●	The technician must watch the worker throughout testing to ensure:
	o	The worker keeps one eye covered and does not peep past.
	o	That the worker does not lean forward to try and see better.
	o	That the worker does not squint or frown when doing the test.
Write the results down on a piece of paper as the test proceeds until the full test is finished.

Common errors

Testing memory rather than vision because the worker learns the chart.
Allowing the worker to peek – thus using both eyes rather than testing one eye at a time.
Failure to shield the worker’s eyes from extraneous light / glare and shadows.
Failure to obtain the corrected acuity when the applicant wears glasses – not testing with and without glasses.
Failure to note (and where relevant to require the removal of) contact lenses.
Permitting the applicant to view the chart with both eyes when testing and recording either the right or left eye.
Failure to observe the worker’s face to detect squinting and posture changes like leaning forward.
Using the incorrect chart and the corresponding incorrect distance (6m chart at 6m / 3m chart at 3m).

Video 8.1: Assessing Far Visual Acuity – Snellen Chart¹

Video 8.2: Assessing Far Visual Acuity – Snellen Chart²

Let’s practice

Select a partner, decide on the role each person will take (1 technician and 1 worker).

Collect the equipment
Prepare the environment
Take a detailed history
Prepare the worker for the test
Conduct a far visual acuity test using a Snellen or Tumbling E chart.

Near Visual Acuity Testing

Near visual acuity is tested manually with a Jaeger, Rosenbaum or Sloan letter chart or using an electronic visual acuity tester such are a Keystone, Titmus or another automated vision tester. Any abnormalities identified in automated testing indicate that a manual test should be done.

Near Vision Eye Test⁶

All near vision charts are tested with the same procedure below.



Jaeger Chart testing of Near Visual Acuity
●	Each eye must be tested separately, and both eyes together.
●	Test without correction and repeat with glasses or contact lenses after the uncorrected test.
●	Explain test to the worker:
	o	Sit and hold the chart at 35 – 40cm from the eyes.
	o	Cover 1 eye at a time but don’t apply pressure to the eye being covered.
	o	The test starts on the right side or with the better eye first.
	o	The assessment is first done without glasses and then repeated with glasses.
	o	Each eye is tested separately and both eyes together.
	o	Don’t move the chart closer or further away, squint, or peep during the test.
	o	If uncertain or if anything is unclear the test can be stopped.
●	Remember if the worker is unable to read, use the Rosenbaum or Sloan letter chart.
●	The worker must cover the right eye first or if the better eye is known start with the better eye.
●	The technician sits in front of the worker and holds the chart with the worker at 35 – 40cm from the workers eyes.
●	Ask the worker to read the clearest line without squinting. The worker must read the paragraph without hesitation on any words based on what is seen and not reading ability.
●	Ask the worker to read the clearest line without squinting. The worker must read the paragraph without hesitation on any words based on what is seen and not reading ability.
●	Change eyes and repeat ending with testing binocular vision – both eyes are used to see the chart.
		Then change eyes and repeat ending with testing binocular vision – both eyes are used to see the chart.
●	The technician must watch the worker throughout testing to ensure:
	o	The worker keeps one eye covered and does not peep past.
	o	That the worker does not lean forward or move the chart further away to try and see better.
	o	That the worker does not squint or frown when doing the test.
Write the results down on a piece of paper as the test proceeds until the full test is finished.

Common errors

Allowing the worker to squint when reading the chart
Allowing the worker to peek – thus using both eyes rather than testing one eye at a time.
Failure to shield the worker’s eyes from extraneous light / glare / shadows.
Permitting the applicant to view the chart with both eyes when testing and recording either the right or left eye.
Failure to observe the worker’s face to detect squinting and posture changes like leaning forward.
Failure to obtain the corrected acuity when the workers wear glasses – not testing with and without correction.
Failure to note (and where relevant to require the removal of) contact lenses.
Recording the incorrect results.

Video 8.3: Assessing Near Visual Acuity – Sloan Letter Chart³

Video 8.4: Assessing Near Visual Acuity – Rosenbaum Chart³

View video on Youtube

Table 8.1: Conversion Tables for Snellen and Jaeger Chart Results


Snellen Equivalents	Near Vision Equivalents
20/20 = 6/6	20/20 = N.4 / J.1
20/25 = 6/7.5	20/25 = N.5 / J.2
20/30 = 6/9	20/30 = N.6 / J.4
20/40 = 6/12	20/40 = N.8 / J.6
20/50 = 6/15	20/50 = N.10 /J.8
20/60 = 6/18	20/60 = N.12 / J.10
20/70 = 6/21	20/70 = N.14 / J.12
20/100 = 6/30	20/100 = N.18 / J.14
20/200 = 6/60	20/200 = N.36

Let’s practice

Select a partner, decide on the role each person will take (1 technician and 1 worker).

Collect the equipment
Prepare the environment
Take a detailed history
Prepare the worker for the test
Conduct a near visual acuity test using a Jaeger or similar chart.

Colour Vision Testing

Colour vision is tested manually using the Ishihara charts or an electronic visual acuity tester such are a Keystone, Titmus or another automated vision tester. Any abnormalities identified in automated testing indicate that a manual test should be done.

Ishihara Chart testing for Colour Vision


Each eye is tested separately.
If the worker wears glasses for reading (near vision) ask the worker to wear the glasses.
Explain test to the worker:
●	Sit comfortably.
●	The technician holds the chart at 70cm from the eyes.
●	Cover 1 eye at a time but don’t apply pressure to the eye being covered.
●	The test starts on the right side or with the better eye first.
●	Each eye is tested separately.
●	Don’t move the chart closer or further away, squint, or peep during the test.
●	If uncertain or if anything is unclear the test can be stopped.
Remember if the worker is unable to read, the worker can trace the numbers with a finger.
The technician sits in front of the worker and holds the chart with the worker at 70cm from the workers eyes.
Start with plate 1 – this is not part of the test it is used to check whether the worker has understood the instructions. Ask the worker what number is inside the circle. If the worker cannot see number “12”, there is not point continuing with the test, change eyes and repeat. Plates 1 and 24 are control plates – therefore not part of the test.
At plate 11, 12, 13 and 14 ask the worker to trace the lines.
●	No. 2. Workers with normal colour vision will read “8” and those with red-green deficiencies “3”.	No. 2. Workers with normal colour vision will read “8” and those with red-green deficiencies “3”.
●	No. 3. Workers with normal colour vision will read “5” and those with red-green deficiencies “2”.
●	No. 4. Workers with normal colour vision will read “29” and those with red-green deficiencies “70”.
●	No. 5. Workers with normal colour vision will read “74” and those with red-green deficiencies “21”.
●	No.6 - 7. Correctly decipherable by normal subjects, but illegible or hard to read for those with red-green deficiencies.
●	No. 8. Workers with normal colour vision will see a clear “2” but it will be obscure for those with red-green deficiencies.
●	No. 9. Workers with normal colour vision will be unable to read it, but most of those with red-green deficiencies see the figure “2” in it.
●	No. 10. Workers with normal colour vision usually read the figures “16”, but most of those with red-green deficiencies cannot.
●	No. 11. In tracing the winding line between the two x’s, workers with normal colour vision trace the bluish-green line, but the majority of those with color vision deficiencies are unable to follow the line or follow a line different from the normal one.
●	No. 12. Workers with normal colour vision and those with mild red-green deficiencies see the figures “35” but protanopia and strong protanomalia will read “5” only, and deuteranopia and strong deuteranomalia “3” only.
●	No. 13. Workers with normal colour vision and those with mild red-green deficiencies see the figures “96” but protanopia and strong protanomalia will read “6” only, and deuteranopia and strong deuteranomalia “9” only.
●	No. 14. In tracing the winding lines between the two x’s, the normal trace along the purple and red lines. In protanopia and strong protanomalia only the purple line is traced, and in case of mild protanomalia both lines are traced but the purple line is easier to follow. In deuteranopia and strong deuteranomalia only the red line is traced and in case of mild deuteranmalia both lines are traced but the red line is easier to follow Then change eyes and repeat ending with testing binocular vision – both eye are used to see the chart
Repeat the test on the other eye.
Write the results down on a piece of paper as the test proceeds until the full test is finished.

Common errors

Testing memory rather than vision as the worker learns the chart
Failure to shield the workers eyes from extraneous light / glare / shadows.
Permitting the applicant to view the chart with both eyes when testing and recording either the right or left eye.
Using photocopied, laminated charts.

Video 8.5: Assessing Colour Vision – Ishihara Chart⁵

Video 8.6: Assessing Colour Vision – Ishihara Chart – watch the first 4 minutes only⁶

Let’s practice

Select a partner, decide on the role each person will take (1 technician and 1 worker).

Collect the equipment
Prepare the environment
Take a detailed history
Prepare the worker for the test
Conduct a colour vision test using an Ishihara chart.

Visual Fields Testing

Testing visual fields test peripheral vision which detects movement best. No charts are used in this assessment when testing manually, they can be tested on automated vision testers. Any abnormalities identified in manual testing indicates that an automated test should be done. Abnormalities in peripheral vision can indicate diseases like macular degeneration, glaucoma, and conditions of the nervous system like stroke, tumours, and head injuries. The SASOM guideline^7,8 outlines the use of the Novissphere when testing visual fields, it is suggested that technicians and clinicians wanting to use this method refer to the SASOM Guideline No 36⁸ when it is explained and demonstrated through pictures. Video 8.7 below demonstrates how to conduct the Novissphere visual fields test.

Video 8.7: Assessing Visual Fields – Using a Novissphere⁷

Visual Fields Testing by Confrontation


	Prepare the environment.
	The room must be lit enough to conduct the test which is usually normal clinic lighting.
	Follow the normal hygiene practices.
	Have the patient remove hats or anything that could interfere with their peripheral vision above the eyes and on the side of the head.
	Remove glasses.
	Explain test to the worker:
	●	Sit and cover 1 eye with the hand.
	●	Cover 1 eye at a time but don’t apply pressure to the eye being covered.
	●	The test starts on the right side or with the better eye first.
	●	The assessment is done without glasses.
	●	Both eyes are tested first and then each eye is tested separately.
	●	Look straight ahead and don’t move the eyes at all to change direction.
	●	If uncertain or if anything is unclear the test can be stopped.
	Sit approximately 1m away and directly in front of the worker. Adjust the seat height until the technician and the worker are at eye level – the knees should almost be touching.
	Start off with gross assessment first
	●	Ask the worker to look with both eyes directly the technician.
	●	Ask the worker whether he / she can see the whole face or is it hazy or blurred around the edges.
	To test the right eye
	●	Instruct the worker to stare directly at the left eye of the technician.
	●	The worker covers the left eye with the left hand.
	●	The technician loses his / her right eye with the right hand.
	●	Check – the technician’s right eye and the worker left eye are closed.
	●	The technician raises his / her right hand to the inferior temporal edge of the peripheral vision halfway between the technician and the patient, hold up 1, 2, or 5 fingers.
	●	Using only 1, 2, and 5 fingers helps to make the number more easily to distinguished by the worker.
	●	The technician asks the worker patient how many fingers can be seen.
	●	Compare the peripheral vision of the worker to that of the technician.
	Repeat the above steps for the right eye.
	Write the results down on a piece of paper as the test proceeds until the full test is finished.
	To test the right and left eyes corner peripheral vision
	●	Instruct the worker to stare directly at the left eye of the technician.
	●	The worker covers the left eye with the left hand.
	●	The technician closes his / her right eye with the right hand.
	●	NOTE – the technician’s right eye and the worker left eye are closed.
	●	The technician brings his / her right hand holding a WHITE object in at an angel from above and behind the ear asking the worker to say “Stop” when the object is first seen. Repeat this on the opposite side again.
	●	The technician brings his / her right hand holding a WHITE object in at an angel from the knee towards the stomach asking the worker to say “Stop” when the object is first seen. Repeat this on the opposite side again.
	●	See the picture above.
	●	Compare the peripheral vision of the worker to that of the technician.
	Write the results down on a piece of paper as the test proceeds until the full test is finished.

Video 8.8: Assessing Visual Fields – by Confrontation⁸

Common errors

Incorrect positioning of the worker.
If the worker does not focus on the eye of the technician during the assessment.
Failure to shield the workers 1 eye therefore not testing peripheral vision.
Testing the incorrect eye and recording the results incorrectly.
Allowing the worker to wear glasses.

Let’s practice

Select a partner, decide on the role each person will take (1 technician and 1 worker).

Collect the equipment
Prepare the environment
Take a detailed history
Prepare the worker for the test
Conduct a peripheral vision test by confrontation.

Depth Perception Test – Steriopsis

Depth perception testing without an automated vision screener includes threading beads, pouring a glass of water, the finger-nose test or the horizontal Lang 2-pencil test^22,8. If a client has monocular vision, he/she should still be tested for depth perception and the worker may still have normal depth perception. An electronic visual acuity tester such are a Keystone, Titmus or another automated vision tester assesses depth perception using dots and insects.

The Horizontal Lang 2-pencil test⁸:


Prepare the environment.
The room must be lit enough to conduct the test which is usually normal clinic lighting.
Follow the normal hygiene practices.
Have the patient remove hats or anything that could interfere with vision above the eyes and on the side of the head.
Remove glasses
Explain test to the worker:
●	Sit and cover 1 eye with an occlude.
●	Cover 1 eye at a time but don’t apply pressure to the eye being covered.
●	The test starts on the right side or with the better eye first.	The test starts on the right side or with the better eye first.
●	The assessment is done without glasses.
●	The worker must match the tip of the pencil in his / her hand to the one in the technician’s hand.
●	If uncertain or if anything is unclear the test can be stopped.	If uncertain or if anything is unclear the test can be stopped.
Sit in front of the worker facing each other at eye level .
The technician holds the pencil at approximately 40cm away and directly in front of the worker.	The technician holds the pencil at approximately 40cm away and directly in front of the worker.	P
The technician hold up a pencil or similar object horizontally in front of the worker.
Instruct the worker to tap the end of a his / her pencil on the top of the one held by the technician – see picture.
The technician must observe the workers eyes and the accuracy of the closing process as well as the precise movement of the pencils.
Repeat the above steps for the left eye and then again with both eyes uncovered.
Repeat the above steps performing the test with the pencils placed vertically.
Write the results down on a piece of paper as the test proceeds until the full test is finished.	Write the results down on a piece of paper as the test proceeds until the full test is finished.	P

Video 8.9: Assessing depth perception – Horizontal Lang 2-pencil test⁹

Other depth perception methods

Threading beads onto a string, threading a large needle or pouring water from a jug into a cup are all alternative methods. With all these methods block 1 eye at a time and keep the objects at least 40cms away from the worker.

Common errors

Incorrect positioning of the worker.
If the worker performs the horizontal assessment and the technician performs the vertical test.
Failure to shield the workers 1 eye.

Let’s practice

Select a partner, decide on the role each person will take (1 technician and 1 worker).

Collect the equipment.
Prepare the environment.t
Take a detailed history.
Prepare the worker for the test
Conduct a depth perception test using any of the 3 methods.

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