1.  Technical Field
The present disclosure relates to a display apparatus in which an image can be overlapped with an external field to be observed. 
2.  Description of Related Art
Unexamined Japanese Patent Publication No.  2010-145721 discloses a head mounted display (HMD) using a combiner which uses a film formed on a flat transparent base material and having reflection and transmission characteristics, transmits external light and reflects image light from a display device in a direction of a pupil of an observer at the same time, and overlaps an image with an external field to enable observation. 
As the combiner, a diffractive reflection type holo-graphic optical element (HOE) having wavelength-selectivity as a reflection and transmission film on a flat surface is disclosed.