//24-bpp P6 PPM solution:
image:=File("lena.ppm","rb").read();
start:=image.find("\n255\n")+5;   // Get sizeof PPM header

foreach n in ([start..image.len()-1,3]){  //  Transform color triplets
   r,g,b:=image[n,3];  // Read colors stored in RGB order
   l:=(0.2126*r + 0.7152*g + 0.0722*b).toInt(); // Derive luminance
   image[n,3]=T(l,l,l);
}

File("lenaGrey.ppm","wb").write(image);