The optical radiometric calibration method based on correlated photons generated by spontaneous parametric down-conversion (SPDC) is a promising method,because it is based on the basic physical phenomena and does not need radiometric traceability.At the present,the SPDC calibration technology is applicable for the photon-counting detector with a highest calibration accuracy of 0.18%.More and more researchers are making effort to improve its calibration accuracy and expand its applicable targets to analog detectors.Correlated photon circles have the high photon rate characteristic.The photon rates of signal and idle light can be improved significantly by using correlated photon circles in calibration.It has very important significance to study the time-correlation characteristic of correlated photon circles. In this paper,the spatial distribution of emission angle of the broadband (450-1000 nm) correlated photons generated by type-I spontaneous parametric down-conversion is presented through the theoretical calculations.The ZEMAX optical software is used to design the optical system which can receive the broadband correlated photon circles and remove the pump laser which is the main source of stray light.The 355 nm wavelength laser is used to pump -barium borate (BBO) crystal to generate the broadband correlated photon circles.A low-light-level CMOS camera is used to adjust and align the opt-mechanical system.Finally,the 430 nm-860 nm correlated photon circles are received by the optical system and detected by the low-light-level CMOS camera.An experimental measurement system is established to measure the time-correlation characteristics of 685 nm and 736.89 nm correlated photon circles.Using 1.0 mW continuous-wave laser to pump BBO crystal,a coincidence peak is observed evidently.The results show that the correlated photon circles generated by type-I SPDC have the characteristics of time-correlation and high photon rates.It can be conductive to improving optical radiometric calibration accuracy based on correlated photons and promoting the development and application in analog detector calibration.