I am using Google Earth Engine (JavaScript) to calculate brightness temperature by converting at-satellite radiance of Landsat 8. In literature the formula
`T = K2 / (ln((K1/L)+1) -273.15` (`T`: brightness temperature; `K1,K2`: calibration constants 1 and 2; `L`: at-satellite radiance)
is used.

My code:

```
var bt_calc5 = function(image) {
  var bt_calc = image.expression(
    'K2 / (Math.log(K1 / L + 1) -273.15)', {
      'L' : image.select('TOAThermalRadiance')
    });
  return image.addBands(bt_calc);
};

```

It should add a brightness temperature band to all images of my collection, but results in `Expression parse error` which is why I think the syntax is not correct. I took the `Math.log` expression for logarithmus naturalis from the JavaScript documentation webpage. How do you formulate logarithms in Earth Engine?