VisionC

VisionC allows to measure low and high contrast visual acuity at far, intermediate and near distances.

Documentation

1.- First steps

1.0.- About auto-brightness and change brightness prevention

For testing vision it is mandatory to switch off auto-brightness in order to maintain the background luminance constant for any environmental light used. The screen brightness can change if you put an App in second plane or if you put the iPad in the sleep mode. This can result in testing vision in different conditions of background brightness. However, from version 2.0 VisionC automatically detects when screen brightness has changed and modifies the brackground brightness to the stored in the App each time that you use the App for testing vision.

1.1.- How to measure Visual Acuity with the ETDRS?

Indepently you measure Far, Intermediate or Near vision the ETDRS procedure is the same:

  1. Read only one letter per row. For instance, first letter starting from the left or from the right and go down after each correct answer
  2. After the first failure, go to the previous line and ask for reading the five letters
  3. If you get ≥ 3/5 correct answers go down one row and ask again the five letters
  4. If you get <3/5 correct answers go up one row and ask again the five letters
  5. Visual acuity will be the last line for which the patient got ≥ 3/5 correct answers

Here you have an example. First you start reading only the first letter and going down up to the first wrong answer

 

As the first wrong answer was at 0.6 logMAR, I ask to the patient for reading the previous line of 0.7 logMAR

After asking for the five letters, the patient was right on three, therefore I ask for reading the complete next line of 0.6 logMAR

In the 0.6 logMAR line the patient got<3 correct answers, therefore the visual acuity of the patient was the last line on which three correct answers were obtained (0.7 logMAR)

 

 

 

1.2.- How to test Far Vision?
Select the presentation distance in the FAR picker -> Press Go below Far-> Confirm

To measure far distance visual acuity you have first to select the distance in the FAR picker view, then put the iPad at the selected distance from the eyes and start to measure with the ETDRS. A confirmation message will appear, please ensure that the iPad is at the distance figured at the confirmation message. The last selected distance will appear when you restart the App.

1.3.- How to test Intermediate Vision?
Select the presentation distance in the INTERMEDIATE picker -> Press Go below Intermediate -> Confirm

To measure intermediate visual acuity you have first to select the distance in the INTERMEDIATE picker view, then put the iPad at the selected distance from the eyes and start to measure with the ETDRS. A confirmation message will appear, please ensure that the iPad is at the distance figured at the confirmation message. The last selected distance will appear when you restart the App.

1.4.- How to test Near Vision?
Select the presentation distance in the NEAR picker -> Press Go below Near -> Confirm

To measure near visual acuity you have first to select the distance in the NEAR picker view, then put the iPad at the selected distance from the eyes and start to measure with the ETDRS. A confirmation message will appear, please ensure that the iPad is at the distance figured at the confirmation message. The last selected distance will appear when you restart the App.

1.5.- How to change the Chart to prevent memorization?
Select RE, LE or BE -> Press Go on the desired distance -> Confirm

The App comes with a different set of letters in order to avoid memorization. Therefore either letters for distance, intermediate and near or right, left and both eyes differ to prevent memorization

1.6.- How to change the notation between logMAR, decimal and Snellen?
Select Settings at the top right -> Select the Notation -> Close

The visual acuity value appears at right and left sides of each row, you can select the notation that will appear on the chart

1.7.- How to change the Contrast for measuring Low Contrast Visual Acuity?
Select the Contrast Percentage -> Use the Test at the Desired Distance

Low Contrast visual acuity can be measured with the VisionC test, you only have to select the desired percentage

2.- Luminance Properties and Gamma Correction

2.1.- Why the app is not available for Android devices?

This is a common question from our users. The answer is because measurement of any type of visual skill related to Contrast such as Contrast Sensitivity Function or Low Contrast Visual Acuity requires from a previous characterization of the display in order to correct the gamma function. In 2016, we found that obtaining an average from several iPad models for the gamma correction we can obtain a reliable measurement of Contrast Sensitivity without a previous characterization of the display.1 However, there are hundreds of different Android displays which means that it is not reliable to measure contrast sensitivity with an Android device without a previous characterization. Despite VisionC was originally programmed for Android and IOS, we decided to remove the App from the Google Play Store because our first aim is to create the most realiable Apps to measure visual function.

  1. Rodríguez-Vallejo M, Monsoriu JA, Furlan WD. Inter-display reproducibility of contrast sensitivity measurement with iPad. Optom Vis Sci. 2016;93(12):1532-1536.
2.2.- What background brightness do I have to select?

In the iPad you can select the percentage of brightness of the screen, however you need to know what is the background luminance for that brightness. The relationship between the percentage of brightness and the luminance can vary between models and has been reported by Livingstone et al.1 For example, tablets at 75% brightness varied from 165–202 cd/m2, and charts ranged from 270–418 cd/m2 which means a less variation between different iPad models than different clinical ETDRS charts.1 You can also found relationships between brightness and luminance in other works.2–5

The international standard normative recommends a background luminance from 80 cd/m2 to 320 cd/m2 and close to 200 cd/m2 for measuring visual acuity.6 However, this luminance differs from the value of 85 cd/m2 reported in some consensus for clinical trials.7 According to this, our recommendation is that you use around 42% of brightness for measuring at around 85 cd/m2 or 82% of brightness for measuring around 200 cd/m2. If you have the PICO device you can select exactly the brightness you need for a particular value of luminance.

Very important !!, do not forget to Switch Off the Auto-brightness from SETTINGS -> GENERAL -> ACCESSIBILITY -> DISPLAY ACCOMMODATIONS -> AUTO-BRIGHTNESS -> OFF

  1. Livingstone IA, Tarbert CM, Giardini ME, Bastawrous A, Middleton D, Hamilton R. Photometric compliance of tablet screens and retro-Illuminated acuity charts as visual acuity measurement devices. PLoS One. 2016;11(3):e0150676.
  2. Rodríguez-Vallejo M, Monsoriu JA, Furlan WD. Inter-display reproducibility of contrast sensitivity measurement with iPad. Optom Vis Sci. 2016;93(12):1532-1536.
  3. Aslam TM, Murray IJ, Lai MYT, et al. An assessment of a modern touch-screen tablet computer with reference to core physical characteristics necessary for clinical vision testing. J R Soc Interface. 2013;10(84):20130239.
  4. Tahir HJ, Murray IJ, Parry NR a, Aslam TM. Optimisation and assessment of three modern touch screen tablet computers for clinical vision testing. PLoS One. 2014;9(4):e95074. doi:10.1371/journal.pone.0095074
  5. Bodduluri L, Boon MY, Dain SJ. Evaluation of tablet computers for visual function assessment. Behav Res Methods. 2017;49(2):548-558. doi:10.3758/s13428-016-0725-1
  6. ISO 10938:2016. Ophthalmic optics. Chart displays for visual acuity measurement. Printed, projected and electronic.
  7. MacRae S, Holladay JT, Glasser A, et al. Special Report: American Academy of Ophthalmology Task Force Consensus Statement for Extended Depth of Focus Intraocular Lenses. Ophthalmology. 2017;124(1):139-141. doi:10.1016/j.ophtha.2016.09.039

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