Bubble sheets without a scanner machine: how phone grading works
Optical mark recognition used to need special forms and a reading machine. How a phone camera reads filled-in circles today, what the corner squares and the QR code are for, and why the sheet asks for a pen.
In short: Reading filled-in circles on a form is called optical mark recognition (OMR). It used to need pre-printed forms and a dedicated reading machine. Today a phone camera can do it: the app finds the sheet by its four corner squares, identifies it by a QR code, and checks which circle in each row is filled in. With Grade AI the sheets are printed on any printer, and the questions are on the same page as the circles.
What optical mark recognition is
Optical mark recognition (OMR) is the technology behind "fill in the bubble" forms: exams, surveys, lottery tickets. The reader does not recognise handwriting. It only checks, for each position on the form, whether it is marked or empty. That makes it fast and reliable, as long as it knows exactly where every circle is.
The old way: special forms and a machine
Classic OMR in schools meant buying pre-printed answer forms and feeding them through a reading machine. It works, but it has costs:
- the forms and the machine have to be bought,
- the machine is in one place, often not the classroom,
- the forms are generic, so students copy their answers from a separate question paper,
- the forms have to be matched to students by hand, or students write an ID number.
The phone way: any printer, any camera
A phone app can do the same reading with the camera, on sheets that the app prints itself. Grade AI does it like this:
- Corner squares. Every sheet has four black squares in its corners. The app finds them in the camera image and uses them to straighten the sheet, so it knows where every circle should be, even if the phone is not perfectly level.
- QR code. The code on the sheet says which student it belongs to and which version of the test (A or B) it is. That is why you can scan the pile in any order.
- Circles. For each question the app compares how dark each of the four circles A, B, C and D is with the paper around it, and takes the filled one as the answer.
- Grade. The answers are compared with the key, the points are added up and turned into a grade on your scale, and the result is saved.
The camera image is processed on the phone itself. The photo is not uploaded, not saved to the photo library and not sent anywhere; only the result is stored.
Why the questions are on the answer sheet
Because the app prints the sheets itself, it can put everything on one page: the student's name, the QR code, the questions and the circles. Students answer next to the question instead of copying letters from one paper to another, which removes a common source of mistakes. On A4 there is room for up to 20 questions, on A5 up to 12, and on half of an A4 page up to 10 (two sheets per page, cut in half).
Why a pen, not a pencil
Every Grade AI sheet tells students to fill in the whole circle with a black or dark blue pen, and not to use a pencil. Dark ink gives a clear contrast with the paper, and a pen mark cannot be rubbed out and changed afterwards. The printed instruction says so too: a filled circle cannot be changed. Crosses and ticks are not reliable; the circle has to be filled in.
What makes a scan fail, and how to avoid it
| Problem | What to do |
|---|---|
| The page is curved or held in the hand | Put it on a flat table |
| Several sheets in view | Keep only one sheet in the camera view |
| Phone held at an angle | Hold it straight above the middle of the sheet |
| Heavy shadow across the paper | Use more even light |
| A corner square is folded or covered | Keep the corners clean and flat |
| Sheet from a different test | The app recognises it and does not save it |
Frequently asked questions
Do I need a special scanner?
No. The phone camera does the reading. The sheets print on any printer, in plain black and white.
Can the app read my old bubble sheets?
No. It reads only the sheets it prints, because it relies on their corner squares and QR code.
Does the app recognise handwriting?
No. It reads which circle is filled in. Names are printed on the sheets, so there is no handwriting to read.
Do the photos of the sheets go to a server?
No. The image is evaluated on the phone, and only the result is saved: which answers were marked, how many were correct and the grade.
Try it on your next test.
Set up a class, print the sheets, see how long it takes. Scanning happens on the phone. Building and printing work in the browser too.