Einsatz digitaler Kameras als Leuchtdichtemessgeräte Diplomarbeit Fakultät für Informations-, Medien- und Elektrotechnik der Fachhochschule Köln Institut für Medien und Phototechnik Autor: Helke Gabele Köln Mat.-Nr. 11031898 Referent: Prof. Dr.-Ing. Gregor Fischer Koreferent: Dipl.-Ing. Dietmar Wüller, Image Engineering, Frechen-Königsdorf Köln, im Februar 2006 The Usage of Digital Cameras as Luminance Meters Diploma Thesis Faculty of Information, Media and Electrical Engineering University of Applied Sciences Cologne Institute of Media and Phototechnology Author: Helke Gabele Cologne Mat.-Nr. 11031898 First Reviewer: Prof. Dr.-Ing. Gregor Fischer Second Reviewer: Dipl.-Ing. Dietmar Wüller, Image Engineering, Frechen-Königsdorf Cologne, February 2006 Kurzbeschreibung Titel: Einstaz digitaler Kameras als Leuchtdichtemessgeräte Autor: Helke Gabele Referenten: Prof. Dr.-Ing. Gregor Fischer / Dipl.-Ing. Dietmar Wüller Zusammenfassung: Diese Arbeit stellt ein Verfahren vor, mit dem digitale Kameras unabhängig von der Belichtung als Leuchtdichtemessgeräte eingesetzt werden können. Eine Kalibrierung der Kamera mit Hilfe einer OECF-Messung wird durchgeführt und die Leuchtdichte aus den digitalen RGB-Ausgabewerten der Kamera berechnet. Fehlerquellen, welche die Genauigkeit der Messung beeinflussen können, werden aufgezeigt. Abschließend wird die Anwendung am Beispiel der Untersuchung von Verkehrsunfällen erklärt. Sperrvermerk: Die Arbeit unterliegt keinem Sperrvermerk. Stichwörter: digitale Kamera, Leuchtdichte, OECF-Messung, Belichtungswert Datum: Februar, 2006 Abstract Title: The Usage of Digital Cameras as Luminance Meters Author: Helke Gabele Reviewers: Prof. Dr.-Ing. Gregor Fischer/ Dipl.-Ing. Dietmar Wüller Summary: The thesis presents a method for the usage of digital cameras as luminance meters, independent from the exposure. A calibration of the camera is performed with the help of an OECF measurement and the luminance is calculated with the camera’s digital RGB output values. Error sources, which can limit the accuracy of measurement, are demonstrated. Finally, the application is explained on the example of accident examination. Remark of closure: This thesis is not closed. Key words: digital camera, luminance, OECF-measurement, exposure value Date: February, 2006 Contents 1 Introduction ............................................................................................ 1 2 Basics...................................................................................................... 4 2.1 Light Measurement..........................................................................................4 2.2 Image Formation..............................................................................................5 2.3 Camera Image Processing...............................................................................6 2.3.1 Spectral Sensitivity of an Image Sensor............................................6 2.3.2 Camera Compensation.......................................................................7 2.3.3 White Balance .....................................................................................8 2.3.4 Demosaicing........................................................................................8 2.4 Registration of Luminance on an Image Sensor............................................9 2.5 Photographic Exposure................................................................................. 10 2.5.1 Exposure Control.............................................................................. 10 2.5.2 Exposure Value................................................................................. 11 2.5.3 Exposure Time.................................................................................. 12 2.5.4 Aperture ............................................................................................ 12 3 Test Method..........................................................................................14 3.1 Selected Camera Types ................................................................................ 14 3.1.1 Professional d-SLR: Nikon D2X ....................................................... 14 3.1.2 Prosumer d-SLR: Canon EOS 350D................................................ 14 3.1.3 Prosumer Compact Camera: Nikon Coolpix 8400......................... 14 3.1.4 Consumer Compact Camera: Fuji Finepix F10 .............................. 14 3.2 Luminance Meter.......................................................................................... 15 3.3 Camera Settings............................................................................................ 15 3.4 Calibration ..................................................................................................... 16 3.5 Measurements .............................................................................................. 17 3.5.1 Constancy of Exposure .................................................................... 17 3.5.2 Sensitivity.......................................................................................... 18 3.5.3 Change in Luminance...................................................................... 18 3.5.4 Over and Underexposure................................................................. 19 I 3.5.5 Different Objects in a Normal Light Scene..................................... 19 3.5.6 Calibration of five Additional Cameras ........................................... 20 4 Calculation of Luminance ....................................................................21 4.1 Transformation of Output RGB Data into CIE Y Value ................................ 21 4.2 Calculation with Exposure Value.................................................................. 24 4.3 Problems of Calculation................................................................................ 26 4.3.1 Exif Data............................................................................................ 26 4.3.2 Restriction of the Digital Output Values ......................................... 27 4.3.3 Steps of the Digital Output Values.................................................. 29 4.3.4 Exposure Steps ................................................................................ 29 5 Sources of Error....................................................................................30 5.1 Camera Error ................................................................................................. 30 5.1.1 Exposure Control.............................................................................. 30 5.1.2 Settings of Aperture and Exposure Time........................................ 30 5.1.3 Setting of ISO.................................................................................... 31 5.1.4 Constancy of Exposure .................................................................... 32 5.1.5 Analog/Digital Conversion............................................................... 32 5.1.6 White Balance .................................................................................. 33 5.1.7 Noise ................................................................................................. 33 5.1.8 Colour Transformations ................................................................... 34 5.1.9 Vignetting.......................................................................................... 35 5.1.10 Stray Light ...................................................................................... 37 5.1.11 JPEG-Compression......................................................................... 38 5.2 Measurement Accuracy ................................................................................ 39 5.2.1 Error of Luminance Meter ............................................................... 39 5.2.2 Error of the Luminance Measurement ........................................... 39 5.2.3 Error of the Approximation Curve.................................................... 39 5.2.4 Error of Measuring Objects with a Spot Luminance Meter ........... 40 6 Analysis of the Results.........................................................................42 6.1 Evaluation of the Individual Cameras.......................................................... 42 6.1.1 Nikon D2X......................................................................................... 42 II 6.1.2 Canon EOS 350D ............................................................................. 43 6.1.3 Nikon Coolpix 8400 ......................................................................... 45 6.1.4 Fuji Finepix F10................................................................................ 46 6.1.5 Five Additional Cameras.................................................................. 47 6.2 Determination and Evaluation of the Total Error ........................................ 48 6.3 Interpretation of the Results ........................................................................ 48 7 Use in Practice......................................................................................50 7.1 Features of a Luminance Measure Camera................................................ 50 7.1.1 Measuring Range............................................................................. 50 7.1.2 Number of Measuring Points .......................................................... 51 7.1.3 Measuring Angle............................................................................... 52 7.1.4 Storage of Luminance