Thank you for posting this question. Can I assume you are using a projector to create the image on your activboard? The most likely scenario for this problem is that the projector lamp has deteriorated and needs replacing, or the projector itself requires a professional clean. You will be in the best position to determine which is more likely depending on the age of the lamp and the number of hours it has done and how regularly your projector filters are cleaned, if the unit has cleanable filter. Have a look at these articles on our KB here and here for more information.
Of course, feel free to ring our Tech Support line if you need assistance. We will be able to tell you which type of Projector you have if you can get the serial number from the back of the unit. Your case reference is 00460342.