Meaning of Aspect Ratio in line-scan camera - system design calculation

[M0Reng]

Hi,

i try to google abut this, but not found what is the Meaning of Aspect Ratio when i'm doing all the necessary

calculation in designing a line-scan camera sytem.

i finally calulate every parameter i need to my system (inspecting objects on conveyor)

and i get the n=13 in the equation of magnification (beta), this results that Aspect Ratio is 13/9

this is the calculations:

beta =  90um / ( n * delta_y ) 

where delta_y is the conveyor movement per 1 step of encoder pulse (delta_y = 67um/encoder pulse),

90um is the center-to-center distance of the camera sensor,

and n is a number between 1 to 27 that defines the Aspect Ratio (there is known table for that in the manual of the line-scan camera)

in my calculations, i had to go reverse in the equations, cause i must capture a L (Line of View ) in specific width.

therefore, from the equation :   

L =  sensor_size * (1 / beta )  i get that beta must be equal to beta=0.103329........ ,

(i have the L and sensor_size values , lets skip on its values its not mater for the next calculation)

that means: beta =  90um / ( n * delta_y )  => extracting n from the equation and we get :  n = 13

When i go to the Aspect Ratio table, to the value of n=13,

i get Aspect Ratio  (AR) equal to  13 / 9  where 13 is in the direction of conveyor travel and 9 is in along the axis of the sensor size.

So, if i understand the equations and the concept of all this, the Aspect Ratio give me the "pixel Area" of this system

if the AR is 9/13 => 1:1.4444444....   and the sensor_pixel_size is 10um (from camera specification and manual),

then the Area of 1 pixel (in the "sensor point of view") is :

sensor_pixel_size = 10um X 14.44444um  that means that each pixel in the image

represents Area of 0.09677mm X 0.13866mm in the object  ( 96.77um X 138.66um)

(0.9677mm is from the magnification equation beta=0.103329 => beta is 1:9.677 )

is that true ???

Is that the meaning of Aspect Ratio ?

Thanks for any reply that help me to verify that.

Best Regards,

Moti

[Nate.H]

Dear Moti,

Is your camera manual available online? If so, can you post back and include a link to it? I think you may be mixing up two different definitions:

pixel aspect ratio: The ratio between the physical horizontal and vertical sizes of the region covered by the pixel. An acquired pixel should optimally be square, thus the optimal value is 1.0, but typically it falls between 0.95 and 1.05, depending on camera quality. Does the camera manual just define "pixel size = 10 um"? If so, you should treat it as a square pixel.

image aspect ratio: The ratio between the image width and its height.

You can see an example of this relationship (different image resolutions) here: http://en.wikipedia.org/wiki/List_of_common_resolutions

I believe that the table in your camera manual is giving you the aspect ratio of the actual image (line) acquired. This image will be a certain number of pixels wide and a certain number of pixes tall, depending on your pixel size and your magnification factor.  

So you will have some relationship between the actual sensor and pixel size (known), and the size of the field of view (physical area your image captures). This relationship will involve things like your magnification factor and aspect ratio. 

Hope this helps,

~Nate 

[M0Reng]

Dear Nate,

given the parameters in my first post:

sensor pixel  size is known from the camera manual : 90um

the magnification is the beta=0.103329

given this parameters and all the parametes i list in my first post,

and given L (List of View)  equal to 400mm,

and Aspect ratio of 13/9

What is the physical image size that 1 pixel represent in the image ?

its not easy for me to understand all this because its line-scan camera,

probably there is difference in this camera type (vs the area scan camera)

cause here we have the motion involved, and the speed of the conveyor influence the Aspect ratio of the image? yes ??!! i dont know..

need help.

Thanks,

Moti

[Nate.H]

Dear Moti,

In your first post, you stated that the sensor pixel size was 10um, but now you say 90um. Is the 90um a typo? Which is correct?

Does your camera manufacturer post the camera manual online? Most camera manufacturers do, and it would be very helpful if you could post back with a link to it for me.

You can think of the line-scan and area-scan cameras in much the same way. The linescan camera can be operated in different modes, but the programming interface should be almost identical to an area scan application. You specify the setting, and the driver builds up your images line by line based on those settings, and on trigger signals. You can operate a linescan camera in different modes, including VHA (Variable Height Acquisition). To read more on this, refer to the NI IMAQ User Manual. 

I'll look for your post clarifying the sensor pixel size and containing the camera manual link.

Best Regards,

~Nate

[M0Reng]

Dear Nate,

sorry, my mistake its 10um

manual : www.baslerweb.com/downloads/11745/L304kc_Users_Manual.pdf

Thanks

Moti

[Nate.H]

Dear Moti,

So it looks like once you have your line of view, you can calculate the physical area represented by one pixel by dividing the #pixels by the line of view.

So if line of view = 400mm, and you have 4080 pixels in your sensor line, you would get 4080pixels/400mm= 10.2pixels/mm in the horizontal direction. We know the pixel is square from the camera specs, so you would have an area of ~0.098mm^2 represented by each pixel given your previous calculations for the magnification.

The aspect ratio of 13/9 (n=13) is the aspect ratio of the acquired image at this magnification, which is based on the number of lines needed to move the image from line sensor to line sensor.

Basler offers to help with lens selection for this camera, and I think they would be able to give you a better description of what each of the parameters means in terms of camera setup than I will. I would recommend that you contact them to get more detailed help on your camera setup, and to make sure that you get a lens that will work for you given any working distance restrictions.

Best Regards,

~Nate 

[M0Reng]

Dear Nate,

Robert from Graftek help me to understand the setup requirements for this camera

and he is really wonderful, i admire his knowledge.

Thank you for explain me this issue.

The best thing is to read more about how to configure a line-scan camera with encoder synchronization.

its look like there is a lack in knowledge in this kind of application,

Or people dont want to share their knowledge from some reason...

Thank you.

Moti

[Nate.H]

Dear Moti,

I'm glad that Robert was able to help you out! Thanks for the feedback on where the documentation is lacking for this type of application; I'll see what I can do about getting more information out there to help future customers with a similar application.

Good luck with the rest of your application!

~Nate