Question

我如何通过iPhone Camera计算勒克斯或照度。我已经计算了所有exif数据，如：

key = FocalLength, value = 3.85
key = MeteringMode, value = 5
key = ShutterSpeedValue, value = 4.591759434012097
key = ExposureProgram, value = 2
key = FocalLenIn35mmFilm, value = 32
key = SceneType, value = 1
key = FNumber, value = 2.4
key = PixelXDimension, value = 480
key = ExposureTime, value = 0.04166666666666666
key = BrightnessValue, value = -0.2005493394308445
key = ApertureValue, value = 2.526068811667588
key = Flash, value = 32
key = ExposureMode, value = 0
key = PixelYDimension, value = 360
key = SensingMethod, value = 2
key = ISOSpeedRatings, value = (
        1250
    )
key = WhiteBalance, value = 0

我也阅读了http://en.wikipedia.org/wiki/Light_meter，并且知道Lux的计算方法是 (N*N*C)/tS

Where N is the relative aperture (f-number)
t is the exposure time (“shutter speed”) in seconds
S is the ISO arithmetic speed
C is the incident-light meter calibration constant

我不明白这个值是指什么。 N是键值数据的ApertureValue或FNumber，t是曝光时间或快门速度。什么是C（320-540或250）的值。我将不同组合中的每个相似值应用于此公式但是得到了错误的结果，因为我与一些计算勒克斯值的应用程序进行比较。 我还需要根据照度来测量辐照度。

此外，我还计算了捕获图像的亮度：

UIImage* image = [UIImage imageNamed:@"image.png"];
unsigned char* pixels = [image rgbaPixels];
double totalLuminance = 0.0;
for(int p=0;p<image.size.width*image.size.height*4;p+=4) {
  totalLuminance += pixels[p]*0.299 + pixels[p+1]*0.587 + pixels[p+2]*0.114;
}
totalLuminance /= (image.size.width*image.size.height);
totalLuminance /= 255.0;
NSLog(@"Image.png = %f",totalLuminance);

http://b2cloud.com.au/tutorial/obtaining-luminosity-from-an-ios-camera

在此先感谢。任何帮助将不胜感激。

Answer 1

你真的需要照度的绝对值吗？或者你可以逃避可以相互比较的相对价值，例如已知的值达到一个恒定的比例因子？

如果前者运气不好：exif数据中没有C，并且使用校准程序检索它需要具有已知强度的光源，并且可能在积分球中拍照。

如果是后者，只需将表达式重写为Lux = C *（N * N / St），其中N == FNumber，t == ExposureTime，S == ISOSpeedRatings，设置C == 1（或任意值））

Answer 2

这是我建议的代码：

(void)imagePickerController:(UIImagePickerController *)anImagePickerController didFinishPickingMediaWithInfo:(NSDictionary *)info 
{
    //NSLog(@"%@",info);

    NSDictionary *temp=[info   objectForKey:@"UIImagePickerControllerMediaMetadata"];
    NSDictionary *temp2=[temp objectForKey:@"{Exif}"];

    float fnumber=[[temp2 objectForKey:@"FNumber"]floatValue];
    float ExposureTime=[[temp2 objectForKey:@"ExposureTime"]floatValue];

    NSArray *iosarray=[temp2 objectForKey:@"ISOSpeedRatings"];

    float ISOSpeedRatings=[[iosarray objectAtIndex:0]floatValue];

    int c=80;//you need to calibrate this value

    float illuminanace=c*(fnumber*fnumber /(ISOSpeedRatings*ExposureTime));

    //NSLog(@"fnumber=%f ",fnumber);
    //NSLog(@"ExposureTime=%f ",ExposureTime);
    //NSLog(@"iosarray=%@",iosarray);
    //NSLog(@"ISOSpeedRatings=%f",ISOSpeedRatings);

    NSLog(@"illiminace =%f lux",illuminanace);
}

从Exif Data计算照度

2 个答案: