Factors that can affect oculometer measurements of pupil diameter are: horizontal (azimuth) and vertical (elevation) viewing angle of the pilot; refraction of the eye and cornea; changes in distance of eye to camera; illumination intensity of light on the eye; and counting sensitivity of scan lines used to measure diameter, and output voltage. To estimate the accuracy of the measurements, an artificial eye was designed and a series of runs performed with the oculometer system. When refraction effects are included, results show that pupil diameter is a parabolic function of the azimuth angle similar to the cosine function predicted by theory: this error can be accounted for by using a correction equation, reducing the error from 6% to 1.5% of the actual diameter. Elevation angle and illumination effects were found to be negligible. The effects of counting sensitivity and output voltage can be calculated directly from system documentation. The overall accuracy of the unmodified system is about 6%. After correcting for the azimuth angle errors, the overall accuracy is approximately 2%

Fulton, C. L.

Harris, R. L., Jr.

English

NASA Technical Reports Server

_ NASA Technical Memorandum 81806
7- Error Analysisand Correctionsto Pupil Diameter
i Measurementswith LangleyResearchCenter's
Oculometer
(NASA-T_l-81806) EEROE AliAL]tSIS AliD 180-27075
CORRECT'rOllS_ PUPIL DIABE'_EE HEASUREBEilTS
lIZTH LAHGLEI RESEAI_CB CEbI_I_RIS GCULOBE_EIi
{NASA) 17 p HC AO2/HF A01 CSCL 068 gnclas .:_
63/52 27831
C. L Fulton and R. L Harris,
,_. _'- .,_/ ij
, May 1980
NationalAeronauticsand
SDaceAdministration _
Ham_on,Virginia23665
_._
1980018576
https://ntrs.nasa.gov/search.jsp?R=19800018576 2020-03-21T17:20:30+00:00Z
.... _ . . ,
ERRORANALYSIS AND CORRECTIONSTO PUPIL DIAMETERMEASUREMENTS
WITHLANGLEYRESEARCHCENTER'S OCULOMETER
, Cynthia L. Fulton and Randall L. Harris, Sr.
• SU_t_tARY
Factors that can affect oculometer measurements of pupil diameter are:
horizbntal (azimuth) and vertical (elevation) viewing angle of the pilot;
refraction of the eye and cornea; changes in distance of eye to camera;
illumination intensity of light on the eye; counting sensitivity of scan lines
used to measur_ diameter, and output voltage.
To estimate the accuracy of the measurements, an artificial eye was
designed and a series of runs performed with the oculometer system. When
refraction effects are included, results show that pupil diameter is a parabolic
function of the azimuth angle similar to the cosine function predicted by
theory: this error can be accounted for by using a correction equation, reducing
the error from 6g to 1.5_ of the actual diameter. Elevation angle and
illumination intensity effects were found to be negligible. The effects of
counting sensitivity and output voltage can be calculated directly from gystem
documentation. The error for counting sensitivity is ±.085mm and could be
reduced by increasing the speed of the system's internal clock. The error for
voltage output is ±.025ram and could be lowered by reducing the sensitivity of
the digital to analog converter. The overall accuracy of the unmodified system
is about 6%. After correcting for the azimuth angle errors, the overall
accuracy is approximately 2%.
I NTRODUCTION
l.angley Research Center has been i,vo!ved in measuring pilot scan patterns
_,fort was devoted to the modification: for several ycar_. Laagiey's original _'"
; of connr.erclal oculometer equipment for use in the limited space of simulation
:: and flight cockpits with more recent emphasis on the application of the oculomcter
to data collection and analysis. One parameter measured by the oculometer is
: pupil diameter, and to date little effort has been devoted to its analysis,
itowevvr, recent studies (ref. 1] have shown the pilot's pupil diameter to !f
_, , ircrease durinr a simulated landing as touchdown approaches. Pupil diameter
is influenced by several factors such as ambient illumination intensity, but it
: _ alsn has been .:hewn to be influenced by stress and cognitive workload (ref. 2
and 3). It was recognized (ref. 4) that the pupil diameter as measured by_, G . ,
_- l.angley's oc,,toroter was inaccurate when the eye was looking away from the
oculometer. This causes a foreshortening effect If meaningful results of _"
attempts to relate pupil diameter to pilot cognitive activity are going to be
_ obtained, a better estimate of pupil diameter will be needed. *
Therefore. in an effort to understand and evaluate the errors associated with i, the current pupil diameter measurements made wi h Langley's oculometer, tests
_ and analysis were initiated. Several factors, in addition to the foreshortening
• ,,,too ,_
1980018576-002
( !
, !
J I
I l
i effect due to not looking directly into the oculometer, were investigated, such
as the accuracy of estimating the start and end of the pupil on the television
j raster lines, output voltage accuracy, refraction due to the cornea of theand
eye. These errors were analyzed and methods are recommended for correcting
some of these errors in order to obtain a good measurement of pupil diameter. , !
! SYMBOLS i i
:! F1 focal length of positive lens, mm
F2 focal length of negative lens, mm
LI vidicon to positive lens distance, mm
L2 vidicon to negative lens distance, mm
P distance from eye to camera lens, mm
PDa actual pupil diameter, nn
PDm measured pupil diameter, mm
Y azimuth angle, angle in the horizontal direction
between the line of sight and the electro-optic
head of the oculometer, deg
0 elevation angle, angle in the vertical direction
between the line of sight and the electro-optic
head of the oculometer, deg
EQUIPMENTAND TESTS
In order to evaluate separately the error effects of the oculometer
interface and computer system and the refractive characteristics of a human
eye, a simulated eye without refractive properties was made by placing on a
light box a piece of cardboard over translucent paper. The cardboard had a
large hole representing the pupil and the paper had a small hole which simula-
ted the cornea reflection. The intensity of the light bulb could be varied to
simulate any level of reflected light from the pupil. The light box was
mounted vertically such that it could rotate about vertical axis to change the
azimuth angle (fig. 1). "
The oculometer used in the experiment is described in reference 5,
Appendix A. The electro-optic head of the oculometer system was replaced with
a camera mounted in a tripod 1.7 meters away from the eye, level with the
center of the eye at the beginning of the experiment. The camera had a horiz-
ontal resolution almost as good as the one in the electro-optic head. A zoom
lens was used to adjust the simulated pupil size to correspond to that normally
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encountered in oculometer operation_. The pupil diameter output voltage wo._
read from a digital voltimetcr.
Eight different cases were run. Three had varied light intensities of the
light box; the lowest intensity at which tile oculometer could detect a pupil, _,
the highest intensity at which the oculometer could detect a pupil, and ._ middle _
intensity level. Two runs were made changing the distance from the camera, onet
, at 1.S metecs and one at 1.8 meters, keeping tile illumination at middle inten-
sity. Two runs were made changing the elevation angle of tlle camera to 10° and
20° by raising the tripod and pointing the camera do_n to keep tlle eye in the
video frame. The distance from tile eye to the camera was constant (1.7 meters].
Finally, a run was made combining all three variables; distoncc t,'_s set at 1.5
meters, elevation angle was lO° and illumination intcn.city was high. For each
case, the camera was positioned according to if-at case's specifications, the
light box was rotated from -60 ° to bO° , adjtv-.l lug the intensity for e,_ch angle
to maintain constant video voltage level of _l',e p_-pil.
i
i RESUI,TSAND DISCUSSION
:__
The first three tatters, azimuth angle, elev:ltion angle, and video :"
illumination level, were analyzed by performing experiments. The effects of
refraction were taken from reference 6. Finally, the effect of system charac-
teristics were estimated from system documentation.
Azimuth Angle
The oculometer allows the pilot to move his head within a cubic foot and _ :
rotate his eyes to 30° horizontally [azimuth angles) and -100 to 30° vertically
(elevation angles). Azimuth angle causes the most noticeable effect on the
measured pupil diameter: as much as 18_ decrease in the measured pupil diameter
at 30 ° . Theoretically, if the pupil is an exact circle and no refraction takes
place, the measured pupil diameter is proportional to the cosine of the azimuth o
angle and independent of the elevation angle. The exact equation is:
. _ PDm = PDa cos _u I (P'lla-2sin _ :
where p is the distance from the eye to the camera lens (See figure 2 for a
i * .schematic illustration and derivation of this equation). However, the term
PDa sin P is so close to zero that the equation can be simplified to
, 2p
PDm = PDa cos
3
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The results shown in figure 3 agree with theoretical predictions. This
correction factor reduces the error to about 4°o of the actual diameter between _ ;
azimuth angles of -30 ° and 30 ° . Data corrected in thi.s manner is also shown in
Figure 3. _ =
Since no refraction occurred in this experiment, pupil diameter data from
reference 6 were used to determine the effects of refraction for various azimuth
angles. As seen in figure 4, refraction causes the pupil diameter to be larger
than the theoretical measurements by about 10°,, at an azimuth angle of 30 ° . To
correct the rat_ measurements, a correction equation can be used: i
PDm
PDa-- (1 + .0005691T [ - .0001025u? 2) ;_
This equatioa was found by curve fitting and nondimensionalizing data from i
reference 6, and gives a maximum error of about 2.5_ of the actual diameter i .:
within the azimuth angle range of 30 °. Most of this error seems to be caused
by noisy data at negative azimuth angles. If these points are ignored, the ,
maximum error is less than 1.5"o.
Elevation Angle
For this experiment, the elevation angle had virtually no effect on the
pupil diameter measurements (fig. 5_. For a=imuth angles between -30" and 30°,
the maximum difference between measured value and actual pupil diameter was
about b%. The measurements at 0° appear to have been shifted about SO to the
right--this is attributed to misalignment of the camera. Refraction effects on ,
horizontal pupil diameters due to elevation angles can be estimated by looking
at the vertical pupil diameters of reference 6 as a function of azimuth angle
(fig. 6). The relationship between elevation angle and pupil diameter is very
small and for small angles, less than 30 ° as is the general case of data using
l,angley's oculometer, the error is negligible.
Illumination Intensity ';
The effects of illumination intensity were essentially negligible {fig. 7).
At high and middle intensity, the maximum difference was about 1'6. Between low
and high, the maximum was about 4%. This error is caused by the fact that the
hardware defines the edge of the pupil as the location along the raster line at
which the video level crosses a certain amplitude. Because of the camera re-
sponse characteristics, the pupil edge is not precisely a square wave, but is a
continuous curve. Therefore, as the pupil intensity is increased, this amplitude
criteria is achieved at a more distal location resulting in a slightly larger
pupil. The hardware tries to compensate for this by adjusting the amplitude , :
: criteria to a certain percentage of the peak pupil level, but this still results }_ :
i in the aforementioned 4% error. This error can be minimized by keeping the 1
l intensity level constant and in medium to high range, which will keep this error i
i below 1"o. I "
! .,R;GINALPAGEISOF POORQUALITY
A
1980018576-005
i[ Distance[
ii|_ The effect of distance was also as expected--the pupil diameter is. • proportional to the distance to the camera {fig. 8). This is accounted for in
actual oculometer runs by a remote focusing servo. The accuracy of thefocusing
" servo can be calculated by considering the correction factor built into the
,' system, PDa - MAGx PDm, where MAG is a constant used to convert vidicon values
of pupil diameter into absolute dimensions. _G is inversely proportional to K1,
• a constant: K1 L22 + L2(L1 FI_ + F2(I,1 - F1), assuming L2 has an accuracy
of the input analog to digital converter (1 in 1024) and using typical values
i for F1, F2, and L1, the pupil diameter measurements are accurate to .O07_,_vhich
is neglxgible.
; However, since the oculometer has a depth of field of approximately 2Smm,
it would be possible for the value of MAGto be incorrect. This _ould result
: in an error proportional to the depth of field divided by the distance from the
_ eye to the positive lens. For most typical installations, this error would be
approximately 1_.
Output Voltage
The pupil diameter output voltage comes from a 0 - 10 volt digital to
analog converter. At a sensitivity of 2.54 mm per volt and assuming the system
is accurate to 1 count out of 1024 for maximum output, the pupil diameter vol-
tage is accurate to .025 mm. This error could be cut in half by reducing the
output sensitivity to 1.27 nun per volt. This sensitivity is about the lowest
that can be used and not cause the output voltage to exceed the maximum allowable.
Scan Line Clock Pulse Rate
The effects of counting sensitivity of the scan lines that measure the
pupil diameter can be calculated given the following system characteristics:
the system internal clock operates at 10 _tegahertz, there are 525 scan lines
per camera frame, operating at 30 frames per second, and each raster line length
is equivalent to 39.1 mm at the typical eye location. This results in a
counting sensitivity of 16.7 clock cycles per mm. Assuming that the system is
accurate to two cycles, one at the beginning of the diameter.and one at the end,
the accuracy of the pupil diameter measurem-nts is then V'2(.06)2 = .085 mm. To
_ " improve this accuracy, the speed of the systems internal clock would have to be
' increased. However, this may not be possible with the current system.
' Overall Accuracy
The overall accuracy of the syste_as it is now, can be calculated based on
the sum of squares estimate of each individual error (table I). This gives an
5
,i
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accuracy of 0.20 mm for small pupils (-3 mm) and .50 mm for larger pupils
('8 m). Assuming that the parabolic correction factor eliminates this e-for,
the accuracy of small and larger pupils are, respectively, ±.09 mm and _..12
(table I). Thus, for Langley's oculometer, the limiting accuracy of _up_l
diameter is the system clock pulse rate.
CONCLUSIONS
The results of this study indicate that corrections can be app!ied to the
pupil diameter data in the form of a second order equation involvir_g only
azimuth angle and the measure5 pupil diameter, which essentially _liminates
errors due to the geometry of the measurements and retraction of the eye. The
other errors caused by the oculometer system are essentially fixed and very
little can be done to eliminate them. Some operational methods such as keeping
the intensity in the mid to high range, and keeping close track of image focus
can help reduce these errors. One system modification which may or may not be
practical, is to increase the internal clock rate. And finally, one software
change, rescaling of the pupil diameter output voltage, could reduce its effect
by half. In total, these corrections result in an overall error of about ±.09
to ±.12 mmdepending on the pupil diameter.
REFERENCES
1. Harris, R. L. and btixon, R. W.: Advanced Transport Operation Effects on
Pilot Scan Patterns. Annual Meeting of the Human Factors Society,
October 30 - November 1, 1979.
2. gahneman, D.: Attention and Effort. Englewood Cliffs, New .Jersey.
Prentice-Hall, 1973.
3. Beatty, J.: Pupillometric Measurement of Cognitive Workload. In the 12th
AnnuaI Conference on Manual Control; pp. 135-143, 1976.
4. Krebs, M.; Wingut, J. W.; and Cunningham, T.: Exploration of an Oculometer
Based Model of Pilot Workload. NASA CR-145153, March 1977.
5. Spady, Amos A.: Airline Scan Patterns During Simulated ILS Approaches.
NASATP 1250, October 1978.
6. tlaines, Richard F.: Dimensions of the Apparent Pupil When Viewed at
Oblique Angles. American Journal of Optimalogy, Vol. 68, No. 4, October
1969.
6
1980018576-007
TABLE I - OVERALL OCULOMETERACCUP/_C_ t
!
, uncorrected accuracy corrected accuracy
ERROR SOURCE nun mn
• T
small (-3ram) large C~8mm) small C~3mm) large t~8mm_
j! ..........
_ azimuthangleand
_ _ refraction ±.18 ±.49 0 0
' tI
distance +.03 +.08 _.05 +- - -.08
w
: ! scan lines t.085 t.085 ±.085 ±.085
output voltage +.025- +-.025 ±.012 +-.012
, , ,=,, _L •
total accuracy +-.20 +-.50 +.09- _+.12
7
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Error analysis and corrections to pupil diameter measurements with Langley Research Center's oculometer

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