Porsch, Tobias; Schmidt, Franz

ASSESSMENT OF DAYLIT GLARE PARAMETERS WITH IMAGING LUMINANCE MEASURING DEVICES (ILMD) AND

IMAGE PROCESSING Porsch, Tobias; Schmidt, Franz

2nd CIE Expert Symposium on Appearance - When appearance meets lighting...

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Outline

• What is the aim of this study?• Prior studies & applications

– Veiling glare-spread function (Lv)

– Threshold Increment (TI)

– UGR (Unified Glare Rating)

• Theoretical & practical realisation– Vertical illuminance (Ev)

– Positioning index (p)

– Threshold luminance

– Calculating the Daylight Glare Probability (DGP)

• Tests and verification• Conclusion

Aim

Prior studies & apps

Realisation

Tests

Conclusion


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What is the aim of this study?• Developing an easy to use tool for the assessment of discomfort glare

parameters based on imaging luminance measuring devices (ILMD)

Used hardware:LMK mobile advanced(Canon EOS 450D qualified for measuring luminances)

circular Fisheye lens(Sigma F2.8/4.5mm EX DC)

Used software:LMK LabSoft(luminance image analysis software)

MS Excel Aim


Realisation

Tests

Conclusion


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Prior studies & applications

nvert

V

EcL

Vroadave

LL

TI 8,0

, )(

65

Aim


Realisation

Tests

Conclusion

Veiling glare-spread function (Lv)

Threshold Increment (TI)

UGR (Unified Glare Rating)

= observing anglec = age depending constant

n = 2 (common use)

)25,0

log(82

2

i i

iSi

A p

L

LUGR


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Theoretical & practical realisation

ambientsourcesourceglare LLfXindexglare ;;;);,(__

Lsource = luminance of the glare source

Lambient = luminance of the ambient (adaptation luminance)

Ωsource = solid angle of the glare source to the area it illuminates

, = spatial position of the glare source

( = 0° viewing direction)

Aim


Realisation

Tests

Conclusion

• Most glare indexes bases on classified ‘glare’ luminances LS(ϑ,ϕ) related to its spatial positions (ϑ,ϕ), to its solid angles ΩS and to the state of adaptation of the subject’s eye – i.e. the ambient luminance LA or there caused vertical illuminance EV.


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Aim


Realisation

Tests

Conclusion

• The first task in using ILMDs is to evaluate luminances “as seen” in the image – using high dynamic range algorithms

OECF calibration optical calibration(i.e. vignetting)

absolute calibration


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• Coordinate transformation rules


Aim


Realisation

Tests

Conclusion

• The second approach is to combine the luminance information with the position information of the measured luminance values for extracting additional relevant information – geometrical adjustment.

,, yx

x

yarctan ²)²()( yxfrf

sin


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Aim


Realisation

Tests

Conclusion

x

yarctan

²)²()( yxfrf

ji ,

elightsourcjifor

jijijiLEji

vert

),(

),(cos),(),(,

• The information is used to summarize the solid angle of a detected luminance source and hence to calculate the vertical illuminance out of this data:

• The result of the performed geometrical adjustment and calculation is an index image for the solid angle increment of each pixel:


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• The Position Index P is calculated according to the proposal of the IES for the visual field above and the equations by Iwata and Tokata for the visual field below the line of sight. Aim


Realisation

Tests

Conclusion

• The main focus at the current state of our studies was on the measurement of the Daylight Glare Probability (DGP) by Wienold.

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1018.91087.5

287.1,

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25

i iv

isis

V

PE

L

EDGP


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Aim


Realisation

Tests

Conclusion

• The DGP requires to indicate a luminance threshold level which is needed to define light sources in the field of view as a glare sources or rather as a part of the background.

• Three principle methods were tested:• using the average luminance of

the entire image multiplied by factor x (x = 4)

• using a task zone area for averaging a threshold luminance

• analysing the luminance histogram of the entire image to adjust the threshold level as proposed by Wolf for the UGR assessment

luminance histogram

1

10

100

1000

10000

100000

1000000

0 2000 4000 6000 8000 10000 12000

L (cd/m²)

Pix

el (

co

un

t N

)


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Aim


Realisation

Tests

Conclusion

• To perform all steps before and inside the calculation formula for the DGP a MS Excel based user interface is used. This front-end has direct access to LMK LabSoft image analysis software, via ActiveX.


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Aim


Realisation

Tests

Conclusion


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Tests and verification

Aim


Realisation

Tests

Conclusion

• The major possibility to establish a measurement series for comparisons of the calculated DGP was given with the use of the ‘evalglare’ tool.

• In a first step a software tool was created to convert the 4 byte luminance image format *.pf into 4 byte RGB *.pic format for the Radiance image viewer tool.

• In the next step the ‘evalglare’ tool was used to perform the glare analysis:

>> evalglare -d -c %str%_small.pic -y %str%_small.pic


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Aim


Realisation

Tests

Conclusion

• Simultaneously to the High-Dynamic luminance capturing a comparison measurement for the vertical illuminance with a Luxmeter was made.

• The comparison of the results shows a high correlation of the vertical illuminance for both:– Luxmeter and luminance image calculation

the used solid angle correction is reliable– Luxmeter and the ‘evalglare’ calculation

the image conversion is correct– additionally the correlation of the average

luminances is high

the image conversion is correct

0

200

400

600

800

1000

1200

1400

1600

1800

0 200 400 600 800 1000 1200 1400 1600

vertical illuminance [lx]

ve

rtic

al

illu

min

an

ce

[lx

]Camera/Luxmeter Evalglare/Luxmeter


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Aim


Realisation

Tests

Conclusion

0

100

200

300

400

500

600

0 50 100 150 200 250 300 350 400 450 500

average luminance [ccd/m²]

av

era

ge

lu

min

an

ce

[c

cd

/m²]

Camera/Evalglare


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Aim


Realisation

Tests

Conclusion

• The comparison of the calculated DGP results shows also a high correlation.

15,00%

17,00%

19,00%

21,00%

23,00%

25,00%

27,00%

29,00%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

DGP (ave_evalglare) DGP_ave DGP_hist DGP_task


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Aim


Realisation

Tests

Conclusion

• Case study: situation number 6 shows a higher mismatch of the DGP result can be found.

luminance histogram

1

10

100

1000

10000

100000

1000000

0 1000 2000 3000 4000 5000 6000 7000

L (cd/m²)

Pix

el (c

ou

nt

N)

E vert (LMK)E vert (evalglare)

Lave 90° (LMK)

Lave (evalglare)

48,8 lx 45,8 lx 18,96 cd/m² 18,1 cd/m²

DGP (ave_evalglare) DGP_hist DGP_ave DGP_task

16,56% 19,94% 19,80% 19,88%


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Conclusion

Aim


Realisation

Tests

Conclusion

ILMDs are mainly used for the assessment of glare values. This is currently state of the art and commonly used in a wide field of scientific researches, concerning this topic.

Simplifying the workaround by implementing heuristic identified formulas into image processing algorithms can be a solution for non-experts to identify glare values under real environment conditions.

Problems:• High contrasts between ambient and glare source do generate

scattered light effects in the imaging system of the ILMD• software scattered light correction (available)• use of optimized imaging systems (available)

• Geometrical and photometrical imaging resolution• mostly very small glare sources dependance of the glare

value from the geometrical resolution


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Thank you for your attention!

Documents

Porsch, Tobias; Schmidt, Franz