How to let Tensorflow object detection api use gray image to train(just 1 channel for input tensor)?

I just want to get real time speed when using model trained by tensorflow object detection api, the input tensor has shape[1, width, height,3], it is 3 channels,but I think if I can just use 1 channel to train my model, it just need gray images as input, therefore , this can reduce the computational complexity, which for my app, speed is very important.