Effects of pixel size in line scan . · PDF fileEffects of pixel size in line scan cameras....

Effects of pixel size in line scan cameras.

Jari Löytömäki

JAI Finland

13 November 2015

JAIIs a camera manufacturer with global presence and

operates via own sites and distribution in more

than 40 countries

San Jose Germany MiyazakiYokohamaShanghaiHelsinkiCopenhagen (HQ)

What is JAI doing ?

A unique blend of technologies & competencies

• Area scan and line scan camera technology

• Software engineering

• Image pre-processing techniques

• High-speed interfaces

• Optical knowledge

• Imager and multi-imager technology

CCD/CMOS

2CCD 3CCD 3CMOS 4CCD 4CMOS

4

EFFECT OF PIXEL AREA

• 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 ?

Principle of imaging: one pixel on the target equals one pixel

on the sensor regardless of the pixel sizes.

5

EXAMPLE: ANOTHER LENS TO VIEW THE SAME

• When in focus, the only source

of light for each pixel is the

counterpart pixel on the target.

• 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.

• 4x increase of pixel size results

in increase of collected light

from the same target by the

same ratio.

Pixel size:

20 x 20 µmPixel size:

10 x 10 µm

lens: 28 mm

#F4

Ø = 28/4 = 7 mm

Aperture ≈ 40 mm2

lens: 56 mm

#F4

Ø = 56/4 = 14 mm

Aperture ≈160 mm2

6

EXAMPLE: 10 OR 20 µm SQUARE PIXELS FOR THE SAME TASKWhat happens with the lenses ?

1 2 2047 2048

10

µm

1 2 2047 2048

28 mm lens – F4

Aperture size = 39 mm2

1 meter 1 meter

WD =

0.7 meter

WD =

1.4 meter

28 mm lens – F4

Aperture size = 39 mm2

1 2 2047 2048

56 mm lens – F4

Aperture size = 156 mm2

1 meter

sweep

At half the distance, the

intensity of light is 4x.

Same lens and aperture now

collects 4 times the light.

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.

20

µm

20

µm

WD =

1.4 meter

7

LOWER REQUIREMENTS TO LENS QUALITYObtain better image sharpness when low-cost lenses are used

Target

Standard quality lens

(low LP/mm rating) Lines focussed on imager

10 µm pixels

20 µm pixels

Lines spread across

multiple pixels

causing soft

gray edges.

Lines fit on

individual pixels

for better image

sharpness.

sweep

LARGER PIXELS NEED FEWER LINE PAIRS PER

MILLIMETER (LP/MM) TO RESOLVE DETAILS.AVOIDS THE NEED FOR HIGHER QUALITY

LENSES, KEEPING COSTS DOWN.

8

MAIN BENEFITS OF LARGER PIXEL SIZES

1. LESS LIGHT, LOWER COSTS

2. LESS NOISE

3. SHARPER IMAGES

4. HIGHER MAXIMUM PRODUCTION THROUGHPUT

A simplified list:

20 µm

20 µm

20 µ

m

9

THE MODELS WITH 20 µm SQUARE PIXELS

2K LINE SENSOR

3CMOS 4CMOSMONOCHROME

Sweep SW-2000M-CL

Sweep SW-2000M-CXP

Sweep+ SW-2000T-CL

Sweep+ SW-2000T-CXP2Sweep+ SW-2000Q-CL

Sweep+ SW-2000Q-CXP2

SWEEP

10

EVEN LARGER PIXEL SIZE FOR DUAL SWIRInGaAs imaging technology

Soon to be released new technology:

- 2-channel beam splitter

- 1024 pixels each

- 25 µm square pixels

- 39 kHz

900 nm to… 1600 nm

Short Wave InfraRed (SWIR)

(900nm – 1700nm)

Near InfraRed (NIR)

(750nm – 1100nm)

Visible Light

(400nm – 750nm)

WAVE

SERIESWA-1000D-CL

NEW !

11

GOING FROM 2K to 8K RESOLUTION – WHAT HAPPENS ?What is the difference between pixel sizes on target and light requirements

1 2048

8192

Line 1

Line 2

Line 3

l/4l/4l/4

16x the light is needed due to smaller pixel size

to get the same response per pixel.

1

l

l/4

Line 1

Line 4

Line rate:

80 kHzsweep

Required line rate:

320 kHz (?)

Field of view / pixel size on moving target

2K

8KLine 1

81921

Line rate:

80 kHzl

20 µm

5 µm

4x the light is needed

due to faster scan rate.

8K5 µm

Pixel size on target is

streching as same

distance is imaged.

In practice the quantum

well is smaller with

smaller pixels and the

line rate can not be that

high.

12

HIGHER RESOLUTION AVAILABLE SOON

Model : Mono Colour / Prism Colour / Trilinear

Examples 4K 8K

Pixel size [µm] 7.5 x 10.5 3.75 x 5.78

Line rate, max. 200 kHz 100 kHz

Non-square pixels in first models.

13

NON-SQUARE PIXELS

Example: 10 µm square vs. 10 µm x 15 µm rectangular pixel (same width).

Steady.

1:1 imaging.

One exposure time.

= center of pixel

10 µm 10 µm

20 µm 25 µm

Direction of the movement (belt).

14

NON-SQUARE PIXELS

The original 50% difference in length causes less than 20% more overlapping and only in one dimension.

1:1 imaging.

One exposure time.

Some optical

inaccuracy added.

10 µm 10 µm

20 µm 25 µm

35 µm30 µm

15

BINNING IMPROVES OPTICAL ACCURACY

The optical overlap between the pixels to be binned will all be covered by the new binned pixel.

BINNED

16

SUMMARY (1 of 3).

1. The principles of line scanning in terms of horizontal and vertical resolution do not depend on the

size or shapes of the pixels.

2. The triggering, timings, resolution and dimensions can be set based on the center point of each pixel

regardless of the shape, size or binning of the pixels. The area and shape of the pixel around it can

be thought of as the collecting area of light (image) for that center point.

3. The horizontal resolution (along the line) is the distance between these center points.

4. The vertical resolution is the distance the center point travels during one line scan.

17

SUMMARY (2 of 3).

5. Overlapping of pixels occurs in the vertical direction due to movement during each scan and in both

dimensions due to optical limitations.

6. Rectangular pixels increase vertical overlapping, but clearly less than what the dimensions would

indicate.

7. Binning helps equally well with both square and non-square pixels by hiding the common overlapping.

18

SUMMARY (3 of 3).

8. Larger pixels bring a benefit proportional to the pixel area. This can be used to increase response or

to decrease noise.

9. Use of higher scan rates does not respectively increase the vertical resolution as the scans will start

to overlap each other.

10. Use of low scan rates minimises vertical overlapping with the cost of lower resolution.

Jari Löytömäki

JAI Finland

jjl@jai.com

for seeing the possibilities

THANK YOU