Can you be more specific on what your set up is and what you have available in terms of hardware (and software) for setting up a measuring system for you plants?

Dear Saed Makhool,

To measure the height of a plant through a proxy detection sensor, maybe you need to oriente your research to Lidar sensors.

Best regards,

Ludovic

after many types of sensors failing , image processing using raspberry pi and MATLAB or python to be used...

Thomas Soddemann

disparity using stereo cameras might be helpful to detect height of plants depends on camera position(camera modeling)

disparity in image processing is to find the depth of object in image 

How many plants do you want to measure with single camera?

What kind of system setting are you thinking of? Has it already been set?

What is the expected accuracy?

One option is to:

- use one camera only on a few plants placed in front of a homogeneous background

- if the plants are small you can use telecentric lenses to get an orthographic projection, providing the same magnification at all distances (https://en.wikipedia.org/wiki/Telecentric_lens)

- if you can't use telecentric lenses, you will have to place the plants to the fixed distance from the camera and calibrate the system for the entire possible height range

Then you have to segment each plant from an image. If you have high contrast between the plant and the background and pretty constant lightning conditions, you can use simple threshold or one of the automatic methods such as Otsu (https://en.wikipedia.org/wiki/Otsu%27s_method). If you don't have such ideal conditions, the segmentation task can become quite challenging.

Please provide some more information about the system, the plants, the budget etc. 

Well,

crowded plants in straight lines as shown in the attachment ,

the number of plants isn't important just the heights are .

the robot runs through paths between these lines of plant but the distance isn't exactly fixed from plants .

* the project isn't built till now .

As suggested by Ludovic Journaux, you should check out Lidar.

OK,

the plants are near to the robot (sensor location) about 20cm and don't forget it is self detection , the target (the top) pf the plant is wanted to determine the height , 

as I understood from reading about LIDAR ,its't self detection , the used must orient the device 

in our project , side view is used not top view 

It is doable,

I think one camera on the top of the plants might be sufficient, as the more the plants go up the larger the leafs and barnacles appear in the images, train the system on all those lengths starting from say 10cm up to the camera height, this is the hard way to do it.

Another easy (and more accurate) way is to have a scale  (e.g. Steel Measuring Tape) standing along the plants in front of the camera on the other side of the greenhouse, the camera should be calibrated to see all the tape from 0 to the max height, in between should be your plants, the system read the images and focuses on the tape, using simple OCR system designed for numbers the lower seen part of the tape is read and converted to numbers, this will be the height of your plants, see the attached image.

P.S. the tape color should be of different color from the whole environment to facilitate segmentation 

P.S. if you want to measure multi lengths use same system with multi tapes using the same single camera 

Another solution:

use a blue wall in front of the camera behind the plants,

celebrate the height of the blue wall to be compared with number of seen pixels in the background,

each time, count the blue pixels from top, down to the last seen blue pixel,

convert pixels to CM,

repeat the process to scan the background from left to right, this will give you multi lengths of the plants.