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The Pilot’s eye. A physiological approach to instrument panel legibility

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1 Aerospace Lighting Institute Technology Seminar Los Angeles, March 8-10, 2010 Presented Paper Laura Rossi [email protected] Politecnico di Torino, - Department of Production Systems Torino (Italy) Paola Iacomussi, Giuseppe Rossi [email protected], [email protected] National Institute of Metrological Research (INRIM), - Optical Division Torino (Italy) Many factors affect display legibility in cockpits. Our research aim is to calculate perceived contrast decreasing due to veiling luminance. We focus our attention on this specific aspect, scarcely considered in aeronautical field. Through the study and development of different psychophysical models of vision and the realization of simulation experiments, parameters for the ergonomic and design of safe cockpit have been quantitatively specified. Key words: Luminance contrast, veiling luminance, glare, display legibility. 1. Summary The detection of a displayed message is the first step of the cognitive process to identify its meaning. This step can be psychophysically modelled and evaluated essentially through measurement of contrast both photometric and colorimetric. The intrinsic contrast is a physical parameter that depends on the display characteristic and on the light distribution in the environment. The perceived contrast is the real parameter of interest and it is obtained from the intrinsic contrast considering the veiling luminance evaluated with a mathematical model of the eyes properties. The threshold contrast is the minimum absolute value of the perceived contrast for seeing the object or the message: this value is influenced by the psychophysiology of the eyes and by other influence parameters like the time need to detect the target. During a flight the pilot have several tasks to do in a rapid and not sequential way. This situation and his/her distraction or fatigue can greatly modify the thresholds contrast values. Our work intends to improve the known vision models in the aeronautical field as to define or suggest pre-normative requirements. The Pilot’s eye. A physiological approach to instrument panel legibility
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Aerospace Lighting Institute Technology Seminar Los Angeles, March 8-10, 2010

Presented Paper

Laura [email protected]

Politecnico di Torino, - Department of Production SystemsTorino (Italy)

Paola Iacomussi, Giuseppe [email protected], [email protected]

National Institute of Metrological Research (INRIM), - Optical DivisionTorino (Italy)

Many factors affect display legibility in cockpits. Our research aim is tocalculate perceived contrast decreasing due to veiling luminance. We focusour attention on this specific aspect, scarcely considered in aeronautical field.Through the study and development of different psychophysical models ofvision and the realization of simulation experiments, parameters for theergonomic and design of safe cockpit have been quantitatively specified.

Key words: Luminance contrast, veiling luminance, glare, display legibility.

1. Summary

The detection of a displayed message is the first step of the cognitive process to identify itsmeaning. This step can be psychophysically modelled and evaluated essentially throughmeasurement of contrast both photometric and colorimetric.

The intrinsic contrast is a physical parameter that depends on the displaycharacteristic and on the light distribution in the environment. The perceived contrast isthe real parameter of interest and it is obtained from the intrinsic contrast considering theveiling luminance evaluated with a mathematical model of the eyes properties.The threshold contrast is the minimum absolute value of the perceived contrast for seeingthe object or the message: this value is influenced by the psychophysiology of the eyes andby other influence parameters like the time need to detect the target.

During a flight the pilot have several tasks to do in a rapid and not sequential way.This situation and his/her distraction or fatigue can greatly modify the thresholds contrastvalues.

Our work intends to improve the known vision models in the aeronautical field asto define or suggest pre-normative requirements.

The Pilot’s eye.A physiological approach to instrument panel legibility

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2. Field of application

In display design, Human Computer Interaction (HCI) main aim is to “make displayslegible” [1]. Military and civil aviation standards prescribe specific requirements forcockpit ergonomics and instrument panel design in order to realize perfect legibility andthen set up procedures to verify it.

As said before, the correct detection of an information displayed is the first step ofthe transmission chain of its meaning. Legibility is the basis for subsequent concept asreadability (the ability to understand an information read) or decisional processes (takinga decision according to an information understood) [2].

Readability depends on legibility plus the cognitive and subjective aspects of thehuman-computer interaction. Therefore to increase readability, legibility must beimproved. It depends basically on two factors: the first physical (environment and displaydesign) and the second physiological (the observer’s eye performance).

Display technology in aviation is absolutely in the forefront and wellstandardized, so we will focus on some critical environmental lighting conditions that,due to the physiology of the eye, can compromise vision and consequently reducelegibility.

3. What disability glare is

The ordinary definition of glare sounds as “hindrance to vision by too much light”. TheInternational Lighting Commission (CIE) [3], [4] distinguishes this phenomena inDisability Glare, defined as “glare that impairs the vision of objects without necessarilycausing discomfort” and Discomfort Glare, defined as “glare that causes discomfortwithout necessarily impairing the vision of objects”. We will focus exclusively on thefirst one.

Disability glare is due to the scatter of light within the eye, usually becomingworse with age and also depending from the colour of the iris [5]. There is more than onesource of intraocular light:

intereflection; irregularities in the ocular surfaces; small and irregular changes in refractive index within the ocular media; turbidity of ocular media; fluorescence in the lens.The scattered and fluorescence light results in veiling light superimposed over the normalscene, effectively reducing contrast of objects observed. A suitable analogy would belooking through fog. The increase in scatter of the fog reduces the contrast of objects andmakes them harder to see. This phenomena is called veiling luminance.

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4. Contrast decreasing by veiling luminance

Contrast is the term used to describe differences in luminance between an object and itsbackground.

An environment emitting or reflecting uniform levels of luminous flux havinghomogeneous wavelength composition would create a perception of uniform brightnessand colour when observed. It is only through detection and discrimination of differencesin luminance and colour that the visual system gains any useful information relative tothe condition of the surroundings (size, shape, motion, etc…).Hence, contrast detection and discrimination is the most basic and important visualability (a general relationship between environmental contrast and vision is that, undergiven lighting conditions, the greater the contrast the easier it is to see objects, details,and spatial relationships).

The minimum contrast value permitting the detection of an object at a certaininstant is named threshold contrast. Its value depends moreover on the luminanceadaptation of the observer’s eye, the angular size of the target, if contrast is positive ornegative (it is lower with negative contrast), on the observation time and the observerage. At low light level, as in night vision under 10 cd m-2, the threshold contrastsincreases if the light level decreases, at high light level, as in daylight, its values is lowerand practically constant [6].

As said, veiling luminance, is the physical parameter that, through an empiricalmathematical model, better describes the physiological origin of disability glare.On equal conditions of vision, veiling luminance reduces perceived contrast. So its valuecan descends under the threshold of perception making not legible the informationdisplayed otherwise perfectly detectable.

Standards [7] calculate contrast in two ways, both depending on LT (target luminance)and LB (background luminance) values.

The formula calculating C1 is the commonly used one and adopted in several standards[ST 1], [ST 2].

C1 = LT - LBLB

(1)

C2 usually is the value considered specifically for objects on displays.

C2 = LT LB

(2)

Equation (2) differentiates itself from (1) by –1.

We call C1 and C2 Intrinsic Contrast, inasmuch as depending only on display technicalcharacteristics and environment illuminance [ST 3]. They are a performance parameter of

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a display describing its capability to show a clear character or symbol in different lightingcondition.

When a pilot looks at a display his eyes received light: from the characters or symbols observed and their near background, focused on the

foveal zone (the one involved in character identification); from other framed environment, focused on retina in extra-foveal zone (useful to

give ancillary information); from not framed environment, not geometrically shielded by cockpit layout or

helmet.The last two contributions create disability glare.

However, considering the model of veiling luminance for glare, it’s possible tocalculate the Perceived Contrast i.e. the contrast pilot effectively perceives when heobserves a target in given glare conditions.Adding LVeil both on target and background luminance, in previous formula we obtain:

Cp = LT + LVeil( ) − LB + LVeil( )

LB + LVeil = LT − LB

LB + LVeil(3)

Please notice the fundamental difference between veiling luminance (generatedinside the eye) and luminance of the display surface, due to self emission and reflectionof environmental light. This facet is fully covered by display and lighting environmentdesign standards [ST 1], [ST 2].

At this point, it is interesting to highlight that the following ratio

Rc = Cp

Ci

= LB LB + LVeil

(4)

does not depends on target luminance.

As LVeil increase, Rc goes from 1 to 0 (see Figure 1), Cp decreases and can reach avalue under the contrast threshold rendering the character illegible. If LVeil = L B theperceived contrast is half of the intrinsic one.In this model of vision, to reduce the influence of disability glare it is better to work witha dark character on a bright background. This solution produces a negative contrast thatcould have also the advantage of a lower contrast threshold level [8].

Considering only the veiling luminance due to the light in the field not framed inthe foveal region and neglecting other aspects, like optical aberrations, the best cockpitlayout has devices with backgrounds with dark characters mounted on an instrumentpanel with the most possible dark surface.

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Figure 1: Rc versus veiling luminance LVeil at different background luminance LB values

5. A predictive model

To evaluate the veiling luminance CIE proposes the CIE General Disability GlareEquation [9]:

LVeilEGlare

= 10θ 3 + 5

θ 2 + 0,1pθ

⎣ ⎢ ⎤

⎦ ⎥ ⋅ 1 + A

62,5⎛

⎝ ⎜

⎠ ⎟

4⎡

⎣ ⎢

⎦ ⎥ + 0,025p (5)

where: Eglare is the illuminance in lux on the observer eye due to the glare sources; θ is the angle between the observation direction and the glare source (in degree); p represents the eye pigmentation factor (0 for black eyes, 0,5 for brown eyes, 1,0

for light eyes and 1,2 for very light-blue eyes) A is the observer age in years (in following evaluations we considered a 25 years old

observer).

This model is an elaboration from experimental investigation carried out in severalyears [9] and is applicable for angles θ between 0,1° and 100°Below 0,1° the effects of optical aberrations prevail on disability effects. The introductionof an eye pigmentation factor is important for angular values beyond 30°, as shown inFigure 2.

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Figure 2: The CIE General Disability Glare Equation

The glaring sources can be “single points” like the sun, lamps and luminaries or“large surround sources” like the sky, mountains and external environment.In these last cases, however, it is difficult to correlate the total eye illuminance with thesource directions. A simple solution is to calculate the eye illuminance from theluminance values of the luminous surfaces or volumes in front of the eye.These luminance values can be obtained with ILMDs (Image Luminance MeasurementDevice) or, generally with less accuracy but with more handiness for in-fieldmeasurement, with CCD photographic cameras. Both instruments shall be adequatelycalibrated in luminance.Using a CCD camera, the framed environment is divided in pixel with known direction,luminance and solid angle.

Figure 3: The geometrical calibration of a ILMD and the angle θp used for glare evaluation. εp

and ϕp are the polar and azimuthal angle of pixel p of the CCD array.

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For a given pixel p of luminance Lp, in the direction θp, the dEGlare,p contribution is:

dEGlare,p = Lp dΩp cosθ p (6)

where dΩp is the solid angle of the pixel. This parameter can be easily approximatedconsidering the detector geometry:

dΩp = Ap cosθ p

2

f 2 (7)

where Ap is the pixel area and f the distance between the image nodal point of the lensand the detector surface, practically the focal length of the camera lens.

To improve measurement accuracy the ILMD or the camera used formeasurement shall be also calibrated geometrically, because of lens distortions andcorrections.

The angular dependence of the veiling glare can be highlight by means of agraphical method, as in Figure 4. We took the following picture during a flight on adouble sit glider at an altitude of nearly 4000 m above sea level.

Here, a polar diagram has been superimposed on the pilot visual field centred onthe observed instrument (in this case a digital variometer with a diameter of 70 mm).The peripheral field around 2° (about 35 mm at 1 m of distance that is approximately thefoveal field) is subdivided into sections that are considered as singular glare sources inproducing the same veiling luminance if they have the same luminance.

Figure 4: A graphical method for the veiling luminance evaluation

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The angular relationships of the polar diagram are given in Table 1

Table 1 Angles in the polar diagram of Figure 4 (numbered from “centre” that isthe inner).

Ring

Centre

1 2 3 4 5 6 7 8 9Angle of

opening [ ° ]

2,0 3,0 4,0 5,8 8,0 11,6 16,6 24,0 36,0 56,8

This model clearly shows that the most critical devices are in the upper zone of theinstrument panel, because the number of sectors with high values of luminance (sky andlandscape) is greater.

6. Worst cases simulations

A simple evaluation of the contribution of the veiling luminance can be obtainedconsidering worst-case conditions.

If the sun is not in the field of view, the sky luminance can be as high as 39 kcdm-2 (luminance value of a white cloud as in [ST 1]). We can suppose to have the upperfield at this constant values and the lower field (the instrument panel) at a low constantluminance value (i.e. 6,8 kcd m-2 as in [ST 1] when the sun, rear to the pilot, lights theinstrument panel).

To simplify the evaluation first we consider the contribution of a uniformluminance surround with a black hole of angular radius Φ in the observation direction:

Lveil = 10θ 3 + 5

θ 2 + 0,1pθ

⎣ ⎢ ⎤

⎦ ⎥ ⋅ 1 + A

62,5⎛

⎝ ⎜

⎠ ⎟

4⎡

⎣ ⎢

⎦ ⎥ + 0,0025p

⎧ ⎨ ⎪

⎩ ⎪

⎫ ⎬ ⎪

⎭ ⎪ Φ

110

∫ dEglare

dEglare = LsurrounddΩ = 2π ⋅ Lsurround ⋅ sinθdθ rad = 2π 2

180⎛

⎝ ⎜

⎠ ⎟ ⋅ Lsurround ⋅ sinθdθdegr

(8)

The result is show in Figure 5 where it is clear the great contribution to the veilingluminance of zone near the direction of observation.

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Figure 5: Veiling luminance of a uniform luminance surround versus the angular radius Φ.

Then the two half values obtained from equation (8) with the different surround values ofthe instrument panel and sky are added to obtain the results of Table 2.

Table 2 Worst-case simulation results.

Sky luminance

Instrument panel

Intrinsic Perceived

[ ° ] [ cm ] [ kcd m-2 ] [ - ] [ - ]

0,5 1,75 4,02 19 0,46

1,0 3,49 2,83 19 0,65

2,0 6,98 2,08 19 0,87

0,5 1,75 3,46 19 0,53

1,0 3,49 2,44 19 0,75

2,0 6,98 1,79 19 1,00

0,5 1,75 3,43 19 0,54

1,0 3,49 2,41 19 0,76

2,0 6,98 1,77 19 1,01

Veiling illuminance

Contrast

Angular radius of

the observed instrument

(black hole)

[ kcd m-2 ]

Observed instrument diameter (eye at 1

m)

Luminance

39

6,8

0,5

0,1

10

The example shows that the veiling luminance due to glare may easily decreasethe perceived contrast of the display, at levels that can reduce the possibilities of a correctdetection. In this example the target luminance was 2 kcd m-2 and the backgroundluminance was 100 cd m-2.

A low luminance of the surface nearest the instrument is a key factor to improvevisibility (look at the first three lines of data in Table 2 where, if the instrument diameteris multiply by three, the veiling luminance is halved).

The sky luminance is the other significant contribution in veiling luminance. Acanopy with a low transmittance factor, a solution not always acceptable for otherreasons, can significantly reduce it.Also the shield of the part of the sky could reduce the veiling luminance, but thisapproach has a relatively low impact because of the angular dependence of the CIEmodel.

Other generally less important sources of veiling luminance should be mentioned. Forexample, the light reflected by specular or near specular surfaces in the cockpit, by otheraircraft or scratches on the canopy.

7. Conclusions

The veiling luminance can reduce the legibility of a display even if its intrinsic contrastsatisfies standard requirements. For this reason, the CIE model of disability glare canhelp in the design of instrument panels and airplane cockpit.Veiling luminance can be measured or estimated with enough accuracy for airplaneengineering purpose, also with simple instrument (as a digital camera) and a graphicalapproach as described in this paper.

8. Bibliography

[1] BROWN, C. MARLIN, (1998), Human-Computer Interface Design Guidelines, Intellect Books, 2–3.

[2] BACHFISCHER, G., (2005), Legibility and Readability - A Review of Literature and Research toUnderstand Issues Referring to Typography on Screens and Device Displays, University of TechnologySydney, Technical Report 05.01.

[3] Commission Internationale de l’Eclairage, CIE, (1987), International Lighting Vocabulary, 17.4, 1987.

[4] VOS, J.J., (2003), Reflections on glare, Lighting Res. Technol. 35,2, 2003, pp 163-176.

[5] FRANSSEN, L., et al, (2007), Pupil Size and Retinal Straylight in the Normal Eye, InvestigativeOphthalmology & Visual Science, May 2007, Vol. 48, No. 5.

[6] ADRIAN, W., (1993), The physiological basis of the visibility concept, Proceeding of the 2nd

international symposium on visibility and luminance in roadway lighting, October 26-27 1993, pagg.17-30.

[7] Commission Internationale de l’Eclairage, CIE, (1992), Contrast and visibility, 95:1992.

[8] LAUREN F.V. SCHARFF (2003), Letter identification performance is better for negative contrast thanpositive contrast, Vision Sciences Society Annual Meeting, Sarasota FL, May 13, 2003

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[9] Commission Internationale de l’Eclairage, CIE, (2002), Collection on glare, 146:2002.

--------------------------------------------------------

[ST 1] MIL-HDBK-87213A (2005), Electronically/Optically Generated Airborne Displays.

[ST 2] MIL-STD-1472F (1999), Department of Defense Design Criteria Standard, Human Engineering.

[ST 3] ISO 9241-305, (2009), Ergonomics of human-system interaction. Part 305: Optical laboratorytest methods for electronic visual displays.


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