The Importance of Wavelengths on Optical Designs

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Angela Floyd
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1 1 The Importance of Wavelengths on Optical Designs Bad Kreuznach, Oct. 2017

2 2 Introduction A lens typically needs to be corrected for many different parameters as e.g. distortion, astigmatism, spherical aberration, coma and relative illumination. Another very important parameter is the correction of color aberration, meaning the optimization of the lens for a pre-defined spectral range. Following we present: o o o o what is meant by the correction of color aberration why it is so important what is necessary for a good correction why it may be different for different applications

3 3 Electromagnetic Spectrum WAVE LENGTHS IN MICROMETERS COSMIC RAYS GAMMA RAYS X-RAYS UV RAYS IR RAYS MICRO WAVE TV AM RADIO WAVES VISIBLE SPECTRUM (nanometers) Visible Wavelengths = 400 to 700nm Near Infrared = 700 to 1000nm Short Wave Infrared (SWIR) = 900 to 1800nm

4 4 What Happens Without Optical Correction Longitudinal Color Aberration Each color has its own focal plane. There is a focal shift between the individual colors. Blue Green Red Sensor Lateral Color Aberration Green Red Sensor Each individual color creates an image with a different magnification. This causes color fringes at dark and bright transitions. Blue

5 5 First Basic Optical Correction Single lens element without any correction Blue Green Red Correction of primary axial color by using 2 different lens shapes and glass types. Blue Green Red

6 6 How It Looks Object Image Object Sensor Response

7 7 Get Together White light is seperated in the complete spectrum at the first lens surface. One task of the optical design is to get all the rays back to one single point on the sensor. first surface last surface Sensor Lens

8 8 Hundreds Of Different Glass Types In order to correct the color aberration of a lens the right choice of glass types is very important. There are hundreds of different glass types available, every single one with its own special color characteristics. Very special glasses for color correction are available, often known as ED glass, but they are very expensive (>1000 /kg), very sensitive in manufacturing and often hard to procure.

9 9 Complex Lens Designs For Aberration Correction

10 10 Typical Applications In Specific Spectral Bands 3D-Measurement Very often a laser or a narrow band LED is used for illumination. So the lens is used more or less monochromatically. narrow band, but somewhere in the nm range Traffic Traffic applications very often work with available daylight or/and near infrared flash in order not to distract the car driver when flashing nm + near infrared Color Print Inspection Only the visible light is required to ensure correct color reproduction nm

11 11 Typical Applications In Specific Spectral Bands Food Inspection Defects in e.g. fruits and vegetables are easily detectable when using SWIR illumination SWIR nm Hyperspectral Imaging In order to determine spectral characteristics of objects very often a very wide spectral band is used nm

12 12 The Universal Lens Generally it is possible to design and build a lens which has practically no color shift even in a very wide spectral range. But this would be a very special design with individually selected expensive glasses, requiring a zero tolerance production. Since this is practically not feasible a reasonable spectral performance has to be defined in the lens specification based on the intended use of the lens. Super Achromat with a focus shift < 20µ in the nm range

13 13 Standard Machine Vision Lens A standard lens for Machine Vision is typically designed for the visible range of nm. The longitudinal color aberration is so well controlled that it does not affect the image quality in a negative way. It can be used for the full nm range but also for every small band within this range. If the coating is suitable for infrared light, such a lens might also work for a narrow band application in the NIR - but only in the NIR. There are also standard lenses available which are corrected for the full nm range, the typical spectral sensitivity range of a CCD-/CMOS-sensor. The longitudinal color requires a better correction so that the lens can be used in the visible nm range and NIR simultaneously without re-focussing.

14 14 Examples LED 850nm nm nm suitable for nm without re-focussing suitable for nm or NIR with re-focussing Focus Shift < 200µm Focus Shift < 200µm in both bands

15 SWIR SWIR (Short wave infrared) means the range of 900 1800nm. It does not only require special sensors but also special lens designs and coatings.

15 15 SWIR SWIR (Short wave infrared) means the range of nm. It does not only require special sensors but also special lens designs and coatings. Lenses which are designed for the visible range or even the full nm range usually don t work in the SWIR range. SWIR 2.8/50 9/17/35 Lp/mm M 1: nm XNP 2.8/50 9/17/35 Lp/mm M 1: nm

16 Customized Lenses While standard lenses need to serve several different applications

For a monochromatic or small bandwith application the lens design of a customized lens

Also the coating can be optimized for the small spectral range with optimized

16 16 Customized Lenses While standard lenses need to serve several different applications a customized lens can always be optimized for the specific application. For a monochromatic or small bandwith application the lens design of a customized lens can be much simpler compared to a standard lens. Also the coating can be optimized for the small spectral range with optimized transmission in the required band not only increasing light on the sensor but also reducing ghosting and flare. Standard lens Customized Lens for 660nm

17 17 Influence Of Spectral Bandwidth On Performance Color aberrations are reducing the performance if a lens is used in a wide spectral range. A limitation of the spectral range, e.g. by a high quality bandpass filter, increases very often the image quality. Xenoplan 2.8/50 f/ / 25 / 50 Lp/mm M 1: nm 465nm +/-35nm IFG BP HT

18 18 Coating Not only the optical correction but also the coating is a very important factor already during the design phase of the lens for a specific spectral range. If the coating is not suited to the spectral range the transmission drops and increased stray light will reduce the image quality. Visible ( nm) Near IR ( nm) SWIR ( nm) Single layer coating + +/- - Multicoating Broadband coating SWIR coating -- +/- ++

19 19 Coatings and Stray Light 10 R 7,5 Multi Coating 5 Single Layer 2,5 Broadband nm without stray light Reflection characteristics of different coatings with stray light

20 20 Summary o There is no general solution for all spectral applications. o The right lens needs to be chosen depending on the spectral requirements of the application. o The broader the spectral range the more complex is the lens design and more special, expensive and sensitive glass types are needed for the color correction. o Not only the lens design but also the lens coating has to be suited to the spectral range of the application.

21 21 Thank You For Your Attention We welcome you at our booth!

COLOUR INSPECTION, INFRARED AND UV

COLOUR INSPECTION, INFRARED AND UV TIPS, SPECIAL FEATURES, REQUIREMENTS LARS FERMUM, CHIEF INSTRUCTOR, STEMMER IMAGING THE PROPERTIES OF LIGHT Light is characterized by specifying the wavelength, amplitude