We describe a novel control approach for form accuracy of microlenses with continuous relief by control of exposure threshold. The approach can be achieved by analysis of the exposure distribution on photoresist and by deriving a relationship between the exposure distributions and the internal photoactive compound concentration (PAC) of the photoresist. The feature of parallel PAC curves for a specific value is determined in this way. We then analyze the development process and discover its critical effect on the approach. Finally we establish the relationship between the PAC distribution and the form accuracy of the relief. Not only is the fabricated relief height significantly increased to as high as 100 mum by use of this method, but we also realized effective fine control of the form accuracy of the continuous relief. With this approach we obtained micro- optic elements with 100-mum relief height and a form error (rms value) of less than 1 mum. (C) 2005 Optical Society of America.