Test Background Brightness

Standardization allows reducing the possible bias due to external sources of variation between centers. Using the same test in the same conditions around the world ensures less variation in the global reported clinical results. Background luminance can be a source of bias. We review why using an iPad could reduce the variability in clinical research.

What background luminance is accepted and recommended?

The International Standard Organization (ISO 10938:2016)¹ establishes that visual acuity should be measured with a background luminance ranging from 80 cd/m² to 320 cd/m², considering the recommended value of 200 cd/m² for clinical practice. Thus, charts around the world might theoretically vary in 240 cd/m², however, evidence tells us that despite using commercially available standardized ETDRS charts, fluctuations between retro-illuminated ETDRS chart cabins used in clinical practice might range from 270 to 418 cd/m², much more than the 165 to 202 cd/m² found between several iPads measured at 75% of brightness.² Therefore, it is recommended for clinical research, despite using commercially available charts, you measure the background luminance to be sure about the Luminance you are using.

What impact can have background luminance variation in clinical research?

For testing visual acuity in normal subjects, background luminance does not have a clinically relevant impact. We know that the effect of doubling the luminance level results in the improvement of one letter for the range from 40 cd/m² to 600 cd/m² (test of five letters per row).³ However, in multifocal procedures, the background brightness might have a higher impact on the results. Brezna et al found, in the comparison of visual performance with different bifocal and trifocal multifocal intraocular lenses (background luminance from 0.14 cd/m² to 55 cd/m²), that the Visual Acuity and the Contrast Sensitivity increased linearly with the luminance until a certain saturation above 55 cd/m².⁴ Unfortunately, authors did not explore values above the 55 cd/m².

During the validation process of Multifocal Lens Analyzer 1.0, we explored this issue testing monocularly two samples of subjects implanted with the same Multifocal Intraocular Lens. Sample 1 was tested with an iPad background brightness of 50% (around 116 cd/m², measured with Spyder3Elite) and was compared with Sample 2 tested with an iPad background brightness of 85% (around 250 cd/m², measured with Spyder3Elite). You can see in the Figure below (left plot), which shows the visual acuity defocus curves with the 95% confidence interval, that no significant differences were found in almost all the range. However, for Contrast Sensitivity Defocus Curves (right plot) differences were enhanced. This means that using Multifocal Lens Analyzer at a 70% of brightness without a previous calibration of the display would not result in relevant differences between iPad models since it has been reported that for different iPads at the same screen brightness, the percentage bias from the mean is rarely higher than +/- 15 cd/m2, much less than the >125 cd/m² of the following Figure.

What happens with the Global Consensus and Standard for Clinical Studies?

Unfortunately, the global consensus for Clinical Studies establishes that background brightness should be at 85cd/m² (+/- 5 cd/m²),⁵ and the ANSI Z80.35-2018 describes a range from 85cd/m² to 110 cd/m².⁷  I do not completely know why this standard was chosen in the past coming from the United States because it has the following drawbacks:

• It is at the bottom of the range established by the ISO from 80 cd/m²  to 300 cd/m²,¹ therefore, you only have an allowed tolerance of 5 cd/m² since standard photopic visual acuity should not be measured below 80 cd/m². 
• In some countries, such as the United Kingdom, a minimum value of 120 cd/m² is required, and in Germany 300 cd/m².⁶
• Despite a background luminance of 200 cd/m² (iPad brightness of 75%) we can say you that you do not need to measure your iPad background brightness, if you want to follow the standard of 85 cd/m² we recommend you to use any of the described calibration procedures (PICO or other colorimeters).

We recommend also you measure the conventional chart of your clinic to know what background brightness are you using. Finally, it is very important to know that you can find some differences between colorimeters, for instance, we have found that the old Spyder3Elite underestimates the luminance in comparison to the new SpyderXElite. We recommend you, therefore, to use SpyderXElite (General Retro-illumination) or PICO with PicoDisplay App which has been calibrated according to SpyderXElite measurements.

 

 

What are the settings recommended for different luminance levels if a calibration cannot be performed?

The latest calibration set we performed to establish the average calibration was performed in December 2020 averaging 15 iPads from several generations, since iPad Air 2nd generation to the iPad 7th generation obtaining the following recommended parameters:

• For background luminance of 85 cd/m2:
  • Brightness: 42% Gammas: R: 2.19 G: 2.21 B: 2.24
• For background luminance of 200 cd/m2:
  • Brightness: 75% Gammas: R: 2.18 G: 2.19 B: 2.20
• For background luminance of 300 cd/m2:
  • Brightness: 91% Gammas: R: 2.18 G: 2.20 B: 2.19

 

References

1. ISO 10938:2016. Ophthalmic optics. Chart displays for visual acuity measurement. Printed, projected and electronic.
2. 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.
3. Sheedy JE, Bailey IL, Raasch TW. Visual acuity and chart luminance. Am J Optom Physiol Opt. 1984;61(9):595-600.
4. Brezna W, Lux K, Dragostinoff N, et al. Psychophysical Vision Simulation of Diffractive Bifocal and Trifocal Intraocular Lenses. Transl Vis Sci Technol. 2016;5(5):13.
5. 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.
6. Consilium Ophthalmologicum Universale. Visual acuity measurement standard. Visual Functions Committee, International Council of Ophthalmology, 1984.
7. ANSI Z80.35-2018. Extended depth of focus intraocular lenses for presbyopia. 2018; 29: 318–22.