The benefits of using cameras with large pixel size in line scan vision applications.

Transcription

1 The benefits of using cameras with large pixel size in line scan vision applications. Why to go against the trend and use smaller resolution and larger pixels? Jari Löytömäki JAI Finland 29 October 2014

2 WHY SHOULD I WANT LARGE PIXELS? Questions More pixels or larger sensor size? Does higher resolution mean sharper images? Why would I choose only 2K pixels when much more is available? Can smaller resolution result in better image quality? Can I really provide and afford the extra speed and light that higher resolution requires? 2

3 ... AND SMALLER RESOLUTION? Trade-offs Sensor size has limits: not possible to get best of both worlds. If high resolution is really a must, then only systems of multiple lower resolution cameras could compete with that. In terms of image quality, cameras should actually be classified by the sensor size and not just the number of pixels. Sometimes less can be more: high quality 2K pixels may be more useful than higher amount at lower quality. If 2K resolution is enough, several distinct benefits become available. 3

4 MAIN BENEFITS A simplified list of benefits: 1. LESS LIGHT, LOWER COSTS 20 µm 2. LESS NOISE 20 µm 3. SHARPER IMAGES 4. HIGHER MAXIMUM PRODUCTION THROUGHPUT 20 µm 4

5 HOW TO COLLECT MORE LIGHT? Principle of imaging: one pixel on the target equals one pixel on the sensor regardless of the pixel sizes. Change of pixel size is compensated either by changing the viewing distance or the lens or a combination of both. How do I then get the advantage of larger pixels and the expected better responsivity? How much do I gain? 5

6 ANOTHER LENS TO VIEW THE SAME When in focus, the only source of light for each pixel is the counterpart pixel on the target. Pixel size: 10 x 10 µm Pixel size: 20 x 20 µm That pixel reflects light into all directions. The size of the lens aperture (pupil) determines how much of all that light is captured into the camera and focused onto that single pixel on the sensor. lens: 28 mm #F4 Ø = 28/4 = 7 mm Aperture 40 mm 2 lens: 56 mm #F4 Ø = 56/4 = 14 mm Aperture 160 mm 2 4x increase of pixel size results in increase of collected light from the same target by the same ratio. 6

7 GOING FROM 10 µm TO 20 µm PIXELS What happens with the lenses? 10 µm 28 mm lens F4 Aperture size = 39 mm 2 20 µm 56 mm lens F4 Aperture size = 156 mm 2 28 mm lens F4 Aperture size = 39 mm 2 WD = 1.4 meter WD = 0.7 meter 20 µm WD = 1.4 meter At half the distance, the intensity of light is 4x. Same lens and aperture now collects 4 times the light meter 1 meter meter Keeping the distance, a longer focal length lens can now be used. With the same F-number, the real size of the lens pupil is now 4x and thus each pixel collects 4x the light. sweep 7

8 PIXEL SIZE AND IMAGE QUALITY Shot noise of light. - Light itself is always noisy, specially when observed in short time periods due to its quantized nature: shot noise. - The signal to noise ratio (SNR) of all cameras is limited by the amount of collected photons. - This absolute limit is the SNR from the light itself defined as the square root of the mean number of photons. A 20 µm pixel collects 8x as much light as a 7 µm pixel with the same light strength. t exp,1 : 5 photons t exp,1 = t exp,2 20 µm t exp,2 : 7 photons sweep 8

9 PIXEL SIZE AND IMAGE QUALITY sweep 20 µm pixels 400 µm² = electrons = shot noise 7 µm pixels 49 µm² sweep LARGE QUANTUM WELL CAPACITY A large pixel area can fill up the large quantum well faster. N = 360 ke - N = 45 ke - CAMERAS WITH SMALLER PIXELS TYPICALLY HAVE SMALLER QUANTUM WELLS. SNR = N : 1 9

10 PIXEL SIZE AND IMAGE QUALITY With full quantum wells. Light collecting area SNR Pixel size sweep 20 µm 400 µm² 600:1 Large quantum well capacity 14 µm 196 µm² SWEEP HAS A LARGE S/N RATIO = INCREASED IMAGE QUALITY 10 µm 100 µm² 7 µm 49 µm² 210:1 Typical quantum well capacity 5 µm 25 µm² 3,5 µm 12 µm² 10

11 LOWER REQUIREMENTS TO LENS QUALITY Obtain better image sharpness when low-cost lenses are used LARGER PIXELS NEED FEWER LINE PAIRS PER MILLIMETER (LP/MM) TO RESOLVE DETAILS. AVOIDS THE NEED FOR HIGHER QUALITY LENSES, KEEPING COSTS DOWN. Target Standard quality lens (low LP/mm rating) Lines focused on imager 10 µm pixels Lines spread across multiple pixels causing soft gray edges. Lines fit on individual pixels for better image sharpness. 20 µm pixels sweep 11

12 HIGHER RESOLUTION WITH SAME TARGET SIZE (FOV) What happens if the resolution is increased from 2K to 8K for the same target and with the goal to maintain the same image quality? - The pixel size is now only 1/16 of the original. - The scan rate must be 4 times as fast to maintain the same image ratio at the same speed of the target. 8K 2K Area 16:1 8K 8K 2K 8K 8K Scan rate 4x Need for light: 64x = (8K/2K) 3 12

13 GOING FROM 2K to 8K RESOLUTION WHAT HAPPENS? What is the difference between pixel sizes on target and light requirements Field of view / pixel size on moving target 2K 20 µm Line 1 l Line rate: 80 khz sweep K 5 µm Line x the light is needed due to smaller pixel size to get the same response per pixel l Line rate: 80 khz Pixel size on target is streching as same distance is imaged. 8K 5 µm Line 1 Line 2 Line 3 Line 4 1 4x the light is needed due to faster scan rate l/4 l/4 l/4 l/4 Required line rate: 320 khz (?) In practice the quantum well is smaller with smaller pixels and the line rate can not be that high. 13

14 NEED FOR SPEED AND LIGHT The increase needed to maintain the same image quality with the same imaging target (FOV), when resolution is increased. PIXEL RESOLUTION SIZE AREA SPEED LIGHT 2K 20 µm 400 µm 2 1x 1x 4K 10 µm 100 µm 2 2x 8x 8K 5 µm 25 µm 2 4x 64x The scan rate needs to increase only by the same factor as the resolution. But, the intensity of light should increase to the power of 3 in respect to this ratio. A FACT THAT IS EASILY NOT RECOGNIZED 14

15 CONCLUSIONS 1. Large pixels give you the increase in responsivity that you would expect based on the ratio of areas. 2. Lower lens requirements mean either sharper images or lower costs. 3. Possible to either decrease the viewing distance or to use longer focal length lenses. 4. If the system already runs at very high scan rates, higher resolution may require slowing down the speed of the inspected product. 5. With limited amount of light, same image quality can be maintained at faster line rates when using larger pixels. 6. Better SNR can only be reached by collecting more photons. Large quantum wells benefit from large pixels. 15

16 GLOBAL REACH worldwide operational and commercial presence with distribution to more than 40 countries

17 RESEARCH & DEVELOPMENT a unique blend of technologies & competencies Area scan and line scan camera technology. Analogue-to-digital circuit design. Software engineering. Image pre-processing techniques. High-speed interfaces. Optical knowledge. Imager and multi-imager technology. CCD/ CMOS 2CCD 3CCD 3CMOS 4CCD 4CMOS

18 SWEEP FIRST RELEASE SWEEP Recently released: SW-2000M-CL-80. First monochrome line scan camera from JAI. First in the new series of six models. 18

19 HIGH PERFORMANCE LINE SCAN CAMERA SW-2000M-CL-80 THAT S SWEEP SW-2000M! SWEEP ULTRA SENSITIE Custom CMOS line sensor only at JAI Pixel size: 20 µm x 20 µm unique Resolution: line of 2048 pixels Line rate: bits 19

20 THE NEW MODELS WITH LARGE PIXELS 2K LINE SENSOR MONOCHROME Sweep SW-2000M-CL-80 Sweep SW-2000M-CXP-80 3CMOS Sweep+ SW-2000T-CL-80 Sweep+ SW-2000T-CXP2-80 4CMOS Sweep+ SW-2000Q-CL-80 Sweep+ SW-2000Q-CXP2-80 SWEEP SWEEP+ 20

21 THANK YOU FOR SEEING THE POSSIBILITIES! SWEEP QUESTIONS? BOOTH: 1F52. 21