Alpha-numeric characters are used to identify radar targets on a p.p.i. display for air traffic control. Normally when using medium- to long-persistence phosphor on the display c.r.t., objectional smear results as alpha-numeric characters are moved. A technique for writing alphanumeric characters is described which reduces the effect of smear by use of interlace-written monoscope alpha-numerics and a mix of different coloured phosphors to enhance contrast of data.